diff --git a/doc/src/fix_spring_self.rst b/doc/src/fix_spring_self.rst
index 4453fd61c5..f8354b17d7 100644
--- a/doc/src/fix_spring_self.rst
+++ b/doc/src/fix_spring_self.rst
@@ -13,7 +13,7 @@ Syntax
 
 * ID, group-ID are documented in :doc:`fix <fix>` command
 * spring/self = style name of this fix command
-* K = spring constant (force/distance units)
+* K = spring constant (force/distance units), can be a variable (see below)
 * dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz)
 
 Examples
@@ -22,6 +22,7 @@ Examples
 .. code-block:: LAMMPS
 
    fix tether boundary-atoms spring/self 10.0
+   fix var all spring/self v_kvar
    fix zrest  move spring/self 10.0 z
 
 Description
@@ -42,6 +43,22 @@ directions, but it can be limited to the xy-, xz-, yz-plane and the
 x-, y-, or z-direction, thus restraining the atoms to a line or a
 plane, respectively.
 
+The force constant *k* can be specified as an equal-style or atom-style
+:doc:`variable <variable>`.  If the value is a variable, it should be specified
+as v_name, where name is the variable name.  In this case, the variable
+will be evaluated each time step, and its value(s) will be used as
+force constant for the spring force.
+
+Equal-style variables can specify formulas with various mathematical
+functions and include :doc:`thermo_style <thermo_style>` command
+keywords for the simulation box parameters, time step, and elapsed time.
+Thus, it is easy to specify a time-dependent force field.
+
+Atom-style variables can specify the same formulas as equal-style
+variables but can also include per-atom values, such as atom
+coordinates.  Thus, it is easy to specify a spatially-dependent force
+field with optional time-dependence as well.
+
 Restart, fix_modify, output, run start/stop, minimize info
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 
diff --git a/src/fix_spring_self.cpp b/src/fix_spring_self.cpp
index df00a2ba8c..bc59ff9987 100644
--- a/src/fix_spring_self.cpp
+++ b/src/fix_spring_self.cpp
@@ -21,20 +21,23 @@
 #include "atom.h"
 #include "domain.h"
 #include "error.h"
+#include "input.h"
 #include "memory.h"
 #include "respa.h"
 #include "update.h"
+#include "variable.h"
 
 #include <cstring>
 
 using namespace LAMMPS_NS;
 using namespace FixConst;
 
+enum { NONE, CONSTANT, EQUAL, ATOM };
+
 /* ---------------------------------------------------------------------- */
 
 FixSpringSelf::FixSpringSelf(LAMMPS *lmp, int narg, char **arg) :
-  Fix(lmp, narg, arg),
-  xoriginal(nullptr)
+  Fix(lmp, narg, arg), xoriginal(nullptr), kstr(nullptr)
 {
   if ((narg < 4) || (narg > 5))
     error->all(FLERR,"Illegal fix spring/self command");
@@ -46,8 +49,14 @@ FixSpringSelf::FixSpringSelf(LAMMPS *lmp, int narg, char **arg) :
   energy_global_flag = 1;
   respa_level_support = 1;
 
-  k = utils::numeric(FLERR,arg[3],false,lmp);
-  if (k <= 0.0) error->all(FLERR,"Illegal fix spring/self command");
+  if (utils::strmatch(arg[3], "^v_")) {
+    kstr = utils::strdup(arg[3] + 2);
+    kstyle = NONE;
+  } else {
+    k = utils::numeric(FLERR,arg[3],false,lmp);
+    kstyle = CONSTANT;
+    if (k <= 0.0) error->all(FLERR,"Illegal force constatnt for fix spring/self command");
+  }
 
   xflag = yflag = zflag = 1;
 
@@ -123,6 +132,25 @@ int FixSpringSelf::setmask()
 
 void FixSpringSelf::init()
 {
+  // check variable
+
+  if (kstr) {
+    kvar = input->variable->find(kstr);
+    if (kvar < 0) error->all(FLERR, "Variable {} for fix spring/self does not exist", kstr);
+    if (input->variable->equalstyle(kvar))
+      kstyle = EQUAL;
+    else if (input->variable->atomstyle(kvar))
+      kstyle = ATOM;
+    else
+      error->all(FLERR, "Variable {} for fix spring/self is invalid style", kstr);
+  }
+  
+
+  if ((kstyle == ATOM) && (atom->nmax > maxatom)) {
+    maxatom = atom->nmax;
+    memory->destroy(kval);
+    memory->create(kval, maxatom, "sprint/self:kval");
+  }
   if (utils::strmatch(update->integrate_style,"^respa")) {
     ilevel_respa = (dynamic_cast<Respa *>(update->integrate))->nlevels-1;
     if (respa_level >= 0) ilevel_respa = MIN(respa_level,ilevel_respa);
@@ -162,24 +190,55 @@ void FixSpringSelf::post_force(int /*vflag*/)
   double dx,dy,dz;
   double unwrap[3];
 
+  // reallocate kval array if necessary
+
+  if ((kstyle == ATOM) && (atom->nmax > maxatom)) {
+    maxatom = atom->nmax;
+    memory->destroy(kval);
+    memory->create(kval, maxatom, "sprint/self:kval");
+  }
+
   espring = 0.0;
 
-  for (int i = 0; i < nlocal; i++)
-    if (mask[i] & groupbit) {
-      domain->unmap(x[i],image[i],unwrap);
-      dx = unwrap[0] - xoriginal[i][0];
-      dy = unwrap[1] - xoriginal[i][1];
-      dz = unwrap[2] - xoriginal[i][2];
-      if (!xflag) dx = 0.0;
-      if (!yflag) dy = 0.0;
-      if (!zflag) dz = 0.0;
-      f[i][0] -= k*dx;
-      f[i][1] -= k*dy;
-      f[i][2] -= k*dz;
-      espring += k * (dx*dx + dy*dy + dz*dz);
-    }
+  if ((kstyle == CONSTANT) || (kstyle == EQUAL)) {
+    // update k if equal style variable
+    if (kstyle == EQUAL) k = input->variable->compute_equal(kvar);
+  
+    for (int i = 0; i < nlocal; i++)
+      if (mask[i] & groupbit) {
+        domain->unmap(x[i],image[i],unwrap);
+        dx = unwrap[0] - xoriginal[i][0];
+        dy = unwrap[1] - xoriginal[i][1];
+        dz = unwrap[2] - xoriginal[i][2];
+        if (!xflag) dx = 0.0;
+        if (!yflag) dy = 0.0;
+        if (!zflag) dz = 0.0;
+        f[i][0] -= k*dx;
+        f[i][1] -= k*dy;
+        f[i][2] -= k*dz;
+        espring += k * (dx*dx + dy*dy + dz*dz);
+      }
+    espring *= 0.5;
+  } else {
+    // update kval for kstyle == ATOM
+    input->variable->compute_atom(kvar, igroup, kval, 1, 0);
+    for (int i = 0; i < nlocal; i++)
+      if (mask[i] & groupbit) {
+        domain->unmap(x[i],image[i],unwrap);
+        dx = unwrap[0] - xoriginal[i][0];
+        dy = unwrap[1] - xoriginal[i][1];
+        dz = unwrap[2] - xoriginal[i][2];
+        if (!xflag) dx = 0.0;
+        if (!yflag) dy = 0.0;
+        if (!zflag) dz = 0.0;
+        f[i][0] -= kval[i]*dx;
+        f[i][1] -= kval[i]*dy;
+        f[i][2] -= kval[i]*dz;
+        espring += kval[i] * (dx*dx + dy*dy + dz*dz);
+      }
 
-  espring *= 0.5;
+    espring *= 0.5;
+  }
 }
 
 /* ---------------------------------------------------------------------- */
@@ -213,7 +272,8 @@ double FixSpringSelf::compute_scalar()
 
 double FixSpringSelf::memory_usage()
 {
-  double bytes = (double)atom->nmax*3 * sizeof(double);
+  double bytes = (double)atom->nmax*4 * sizeof(double);
+  if (kstyle == ATOM) bytes += (double)atom->nmax * sizeof(double);
   return bytes;
 }
 
diff --git a/src/fix_spring_self.h b/src/fix_spring_self.h
index f13f2be918..e3220b8cfd 100644
--- a/src/fix_spring_self.h
+++ b/src/fix_spring_self.h
@@ -50,8 +50,11 @@ class FixSpringSelf : public Fix {
  protected:
   double k, espring;
   double **xoriginal;    // original coords of atoms
+  char *kstr;            // name of variable for K
+  double *kval;          // per-atom variable values for K
+  int kvar, kstyle;
   int xflag, yflag, zflag;
-  int ilevel_respa;
+  int ilevel_respa, maxatom;
 };
 
 }    // namespace LAMMPS_NS
diff --git a/unittest/force-styles/tests/fix-timestep-spring_self_atom.yaml b/unittest/force-styles/tests/fix-timestep-spring_self_atom.yaml
new file mode 100644
index 0000000000..e8b35e6455
--- /dev/null
+++ b/unittest/force-styles/tests/fix-timestep-spring_self_atom.yaml
@@ -0,0 +1,77 @@
+---
+lammps_version: 17 Feb 2022
+date_generated: Fri Mar 18 22:18:01 2022
+epsilon: 5e-14
+skip_tests: kokkos_omp
+prerequisites: ! |
+  atom full
+  fix spring/self
+pre_commands: ! |
+  variable kvar atom 10.0
+post_commands: ! |
+  fix move all nve
+  fix test solute spring/self v_kvar xyz
+input_file: in.fourmol
+natoms: 29
+global_scalar: 0.12623705370750438
+run_pos: ! |2
+    1 -2.7045669792379945e-01  2.4912140072031601e+00 -1.6695897908630153e-01
+    2  3.1003572362014503e-01  2.9612290242130319e+00 -8.5466689099875615e-01
+    3 -7.0398410505917419e-01  1.2305522448803927e+00 -6.2777452858703953e-01
+    4 -1.5818137373512378e+00  1.4837442978868092e+00 -1.2538665277734968e+00
+    5 -9.0719266809604937e-01  9.2652365891304722e-01  3.9954299416556610e-01
+    6  2.4832271436421161e-01  2.8312320769893828e-01 -1.2314303818391423e+00
+    7  3.4143518373063453e-01 -2.2645187306940717e-02 -2.5292229406165907e+00
+    8  1.1743540824610306e+00 -4.8863162985819381e-01 -6.3783828665914544e-01
+    9  1.3800508883119953e+00 -2.5274565574966718e-01  2.8353436538531862e-01
+   10  2.0510726036138696e+00 -1.4604027090169940e+00 -9.8323554549077119e-01
+   11  1.7878052123899795e+00 -1.9921835931655048e+00 -1.8890566136917575e+00
+   12  3.0063000763288268e+00 -4.9013323763786637e-01 -1.6231890083142151e+00
+   13  4.0515352007695906e+00 -8.9202569828585798e-01 -1.6399995011867139e+00
+   14  2.6066952690925067e+00 -4.1788633645045253e-01 -2.6633949696742012e+00
+   15  2.9695337662435719e+00  5.5423141568538492e-01 -1.2342076641871542e+00
+   16  2.6747001492056977e+00 -2.4124097322472577e+00 -2.3429048072365732e-02
+   17  2.2153591049025310e+00 -2.0897997214660862e+00  1.1963106355359285e+00
+   18  2.1369701704115056e+00  3.0158507413628213e+00 -3.5179348337213843e+00
+   19  1.5355837136087338e+00  2.6255292355375399e+00 -4.2353987779878857e+00
+   20  2.7727573005678758e+00  3.6923910449610102e+00 -3.9330842459133470e+00
+   21  4.9040128073205524e+00 -4.0752348172959030e+00 -3.6210314709893159e+00
+   22  4.3582355554440877e+00 -4.2126119427287101e+00 -4.4612844196314150e+00
+   23  5.7439382849307670e+00 -3.5821957939275060e+00 -3.8766361295935892e+00
+   24  2.0689243582422914e+00  3.1513346907271247e+00  3.1550389754829422e+00
+   25  1.3045351331492516e+00  3.2665125705842941e+00  2.5111855257434352e+00
+   26  2.5809237402711318e+00  4.0117602605482858e+00  3.2212060529089945e+00
+   27 -1.9611343130357277e+00 -4.3563411931359841e+00  2.1098293115523528e+00
+   28 -2.7473562684513411e+00 -4.0200819932379330e+00  1.5830052163433952e+00
+   29 -1.3126000191359812e+00 -3.5962518039482934e+00  2.2746342468737817e+00
+run_vel: ! |2
+    1  8.1685220941861477e-03  1.6512578512542727e-02  4.7892799147935001e-03
+    2  5.4427456394786321e-03  5.1693257879352533e-03 -1.4414043022813649e-03
+    3 -8.2272458036248362e-03 -1.2923813188884230e-02 -4.0970749471144546e-03
+    4 -3.7660861920462349e-03 -6.5659911420830365e-03 -1.1120922532834726e-03
+    5 -1.1012635909013241e-02 -9.8847866026321157e-03 -2.8391869073674538e-03
+    6 -3.9665990411620729e-02  4.6803722380071487e-02  3.7135522426802389e-02
+    7  9.1016763589152859e-04 -1.0126055720737583e-02 -5.1556610019025714e-02
+    8  7.9043585267658430e-03 -3.3496064544244345e-03  3.4549326598010660e-02
+    9  1.5620907286754389e-03  3.7378245105921431e-03  1.5036774253075934e-02
+   10  2.9193799040059056e-02 -2.9242248165535563e-02 -1.5014281912567770e-02
+   11 -4.7797459644718264e-03 -3.7436196398511232e-03 -2.3410499103477603e-03
+   12  2.2686069875175316e-03 -3.4732729502899497e-04 -3.0627334265471650e-03
+   13  2.7456854188010020e-03  5.8081889921879817e-03 -7.9308949311655092e-04
+   14  3.5223319737918667e-03 -5.7842699330258648e-03 -3.9396805101296825e-03
+   15 -1.8475459117759364e-03 -5.8469790281561471e-03  6.2849983323582511e-03
+   16  1.8676069228413028e-02 -1.3258381729410438e-02 -4.5625616778429308e-02
+   17 -1.2893668780819389e-02  9.7505325833410258e-03  3.7288200735675299e-02
+   18 -8.0065794869105819e-04 -8.6270473288011819e-04 -1.4483040693746142e-03
+   19  1.2452390836051499e-03 -2.5061097119616180e-03  7.2998631010316650e-03
+   20  3.5930060229538143e-03  3.6938860309035470e-03  3.2322732687958995e-03
+   21 -1.4689220366910704e-03 -2.7352129796142532e-04  7.0581624168175334e-04
+   22 -7.0694199254520765e-03 -4.2577148925037030e-03  2.8079117611209205e-04
+   23  6.0446963117617505e-03 -1.4000131614895336e-03  2.5819754846773601e-03
+   24  3.1926367911308686e-04 -9.9445664741642462e-04  1.4999996978363057e-04
+   25  1.3789754526895179e-04 -4.4335894884219599e-03 -8.1808136698604454e-04
+   26  2.0485904035342870e-03  2.7813358633902757e-03  4.3245727149365584e-03
+   27  4.5604120291626942e-04 -1.0305523027244966e-03  2.1188058375789067e-04
+   28 -6.2544520861839200e-03  1.4127711176141612e-03 -1.8429821884806304e-03
+   29  6.4110631535703737e-04  3.1273432719578029e-03  3.7253671105604122e-03
+...
diff --git a/unittest/force-styles/tests/fix-timestep-spring_self_equal.yaml b/unittest/force-styles/tests/fix-timestep-spring_self_equal.yaml
new file mode 100644
index 0000000000..382cc2cc73
--- /dev/null
+++ b/unittest/force-styles/tests/fix-timestep-spring_self_equal.yaml
@@ -0,0 +1,77 @@
+---
+lammps_version: 17 Feb 2022
+date_generated: Fri Mar 18 22:18:01 2022
+epsilon: 5e-14
+skip_tests: kokkos_omp
+prerequisites: ! |
+  atom full
+  fix spring/self
+pre_commands: ! |
+  variable kvar equal 10.0
+post_commands: ! |
+  fix move all nve
+  fix test solute spring/self v_kvar xyz
+input_file: in.fourmol
+natoms: 29
+global_scalar: 0.12623705370750438
+run_pos: ! |2
+    1 -2.7045669792379945e-01  2.4912140072031601e+00 -1.6695897908630153e-01
+    2  3.1003572362014503e-01  2.9612290242130319e+00 -8.5466689099875615e-01
+    3 -7.0398410505917419e-01  1.2305522448803927e+00 -6.2777452858703953e-01
+    4 -1.5818137373512378e+00  1.4837442978868092e+00 -1.2538665277734968e+00
+    5 -9.0719266809604937e-01  9.2652365891304722e-01  3.9954299416556610e-01
+    6  2.4832271436421161e-01  2.8312320769893828e-01 -1.2314303818391423e+00
+    7  3.4143518373063453e-01 -2.2645187306940717e-02 -2.5292229406165907e+00
+    8  1.1743540824610306e+00 -4.8863162985819381e-01 -6.3783828665914544e-01
+    9  1.3800508883119953e+00 -2.5274565574966718e-01  2.8353436538531862e-01
+   10  2.0510726036138696e+00 -1.4604027090169940e+00 -9.8323554549077119e-01
+   11  1.7878052123899795e+00 -1.9921835931655048e+00 -1.8890566136917575e+00
+   12  3.0063000763288268e+00 -4.9013323763786637e-01 -1.6231890083142151e+00
+   13  4.0515352007695906e+00 -8.9202569828585798e-01 -1.6399995011867139e+00
+   14  2.6066952690925067e+00 -4.1788633645045253e-01 -2.6633949696742012e+00
+   15  2.9695337662435719e+00  5.5423141568538492e-01 -1.2342076641871542e+00
+   16  2.6747001492056977e+00 -2.4124097322472577e+00 -2.3429048072365732e-02
+   17  2.2153591049025310e+00 -2.0897997214660862e+00  1.1963106355359285e+00
+   18  2.1369701704115056e+00  3.0158507413628213e+00 -3.5179348337213843e+00
+   19  1.5355837136087338e+00  2.6255292355375399e+00 -4.2353987779878857e+00
+   20  2.7727573005678758e+00  3.6923910449610102e+00 -3.9330842459133470e+00
+   21  4.9040128073205524e+00 -4.0752348172959030e+00 -3.6210314709893159e+00
+   22  4.3582355554440877e+00 -4.2126119427287101e+00 -4.4612844196314150e+00
+   23  5.7439382849307670e+00 -3.5821957939275060e+00 -3.8766361295935892e+00
+   24  2.0689243582422914e+00  3.1513346907271247e+00  3.1550389754829422e+00
+   25  1.3045351331492516e+00  3.2665125705842941e+00  2.5111855257434352e+00
+   26  2.5809237402711318e+00  4.0117602605482858e+00  3.2212060529089945e+00
+   27 -1.9611343130357277e+00 -4.3563411931359841e+00  2.1098293115523528e+00
+   28 -2.7473562684513411e+00 -4.0200819932379330e+00  1.5830052163433952e+00
+   29 -1.3126000191359812e+00 -3.5962518039482934e+00  2.2746342468737817e+00
+run_vel: ! |2
+    1  8.1685220941861477e-03  1.6512578512542727e-02  4.7892799147935001e-03
+    2  5.4427456394786321e-03  5.1693257879352533e-03 -1.4414043022813649e-03
+    3 -8.2272458036248362e-03 -1.2923813188884230e-02 -4.0970749471144546e-03
+    4 -3.7660861920462349e-03 -6.5659911420830365e-03 -1.1120922532834726e-03
+    5 -1.1012635909013241e-02 -9.8847866026321157e-03 -2.8391869073674538e-03
+    6 -3.9665990411620729e-02  4.6803722380071487e-02  3.7135522426802389e-02
+    7  9.1016763589152859e-04 -1.0126055720737583e-02 -5.1556610019025714e-02
+    8  7.9043585267658430e-03 -3.3496064544244345e-03  3.4549326598010660e-02
+    9  1.5620907286754389e-03  3.7378245105921431e-03  1.5036774253075934e-02
+   10  2.9193799040059056e-02 -2.9242248165535563e-02 -1.5014281912567770e-02
+   11 -4.7797459644718264e-03 -3.7436196398511232e-03 -2.3410499103477603e-03
+   12  2.2686069875175316e-03 -3.4732729502899497e-04 -3.0627334265471650e-03
+   13  2.7456854188010020e-03  5.8081889921879817e-03 -7.9308949311655092e-04
+   14  3.5223319737918667e-03 -5.7842699330258648e-03 -3.9396805101296825e-03
+   15 -1.8475459117759364e-03 -5.8469790281561471e-03  6.2849983323582511e-03
+   16  1.8676069228413028e-02 -1.3258381729410438e-02 -4.5625616778429308e-02
+   17 -1.2893668780819389e-02  9.7505325833410258e-03  3.7288200735675299e-02
+   18 -8.0065794869105819e-04 -8.6270473288011819e-04 -1.4483040693746142e-03
+   19  1.2452390836051499e-03 -2.5061097119616180e-03  7.2998631010316650e-03
+   20  3.5930060229538143e-03  3.6938860309035470e-03  3.2322732687958995e-03
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