diff --git a/desc/magnetic_fields/__init__.py b/desc/magnetic_fields/__init__.py
index 094fbc724a..0a8f18abd8 100644
--- a/desc/magnetic_fields/__init__.py
+++ b/desc/magnetic_fields/__init__.py
@@ -1,6 +1,7 @@
 """Classes for Magnetic Fields."""
 
 from ._core import (
+    MagneticFieldFromUser,
     OmnigenousField,
     PoloidalMagneticField,
     ScalarPotentialField,
diff --git a/desc/magnetic_fields/_core.py b/desc/magnetic_fields/_core.py
index 6eee8f3da2..6900d9cbd5 100644
--- a/desc/magnetic_fields/_core.py
+++ b/desc/magnetic_fields/_core.py
@@ -24,7 +24,7 @@
 from desc.optimizable import Optimizable, OptimizableCollection, optimizable_parameter
 from desc.singularities import compute_B_plasma
 from desc.transform import Transform
-from desc.utils import copy_coeffs, flatten_list, setdefault, warnif
+from desc.utils import copy_coeffs, errorif, flatten_list, setdefault, warnif
 from desc.vmec_utils import ptolemy_identity_fwd, ptolemy_identity_rev
 
 
@@ -540,6 +540,85 @@ def save_mgrid(
         file.close()
 
 
+class MagneticFieldFromUser(_MagneticField, Optimizable):
+    """Wrap an arbitrary function for calculating magnetic field in lab coordinates.
+
+    Parameters
+    ----------
+    fun : callable
+        Function to compute magnetic field at arbitrary points. Should have a signature
+        of the form ``fun(coords, params) -> B`` where
+
+          - ``coords`` is a (n,3) array of positions in R, phi, Z coordinates where
+            the field is to be evaluated.
+          - ``params`` is an array of optional parameters, eg for optimizing the field.
+          - ``B`` is the returned value of the magnetic field as a (n,3) array in R,
+            phi, Z coordinates.
+
+    params : ndarray, optional
+        Default values for parameters. Defaults to an empty array.
+
+    """
+
+    def __init__(self, fun, params=None):
+        errorif(not callable(fun), ValueError, "fun must be callable")
+        self._params = jnp.asarray(setdefault(params, jnp.array([])))
+
+        import jax
+
+        dummy_coords = np.empty((7, 3))
+        dummy_B = jax.eval_shape(fun, dummy_coords, self.params)
+        errorif(
+            dummy_B.shape != (7, 3),
+            ValueError,
+            "fun should return an array of the same shape as coords",
+        )
+        self._fun = fun
+
+    @optimizable_parameter
+    @property
+    def params(self):
+        """ndarray: Parameters of the field allowed to vary during optimization."""
+        return self._params
+
+    @params.setter
+    def params(self, params):
+        self._params = params
+
+    def compute_magnetic_field(
+        self, coords, params=None, basis="rpz", source_grid=None
+    ):
+        """Compute magnetic field at a set of points.
+
+        Parameters
+        ----------
+        coords : array-like shape(n,3)
+            Nodes to evaluate field at in [R,phi,Z] or [X,Y,Z] coordinates.
+        params : array-like, optional
+            Optimizable parameters, defaults to field.params.
+        basis : {"rpz", "xyz"}
+            Basis for input coordinates and returned magnetic field.
+        source_grid : Grid, int or None or array-like, optional
+            Unused by this class, only kept for API compatibility
+
+        Returns
+        -------
+        field : ndarray, shape(N,3)
+            magnetic field at specified points
+
+        """
+        coords = jnp.atleast_2d(jnp.asarray(coords))
+        if params is None:
+            params = self.params
+        if basis == "xyz":
+            coords = xyz2rpz(coords)
+
+        B = self._fun(coords, params)
+        if basis == "xyz":
+            B = rpz2xyz_vec(B, phi=coords[:, 1])
+        return B
+
+
 class ScaledMagneticField(_MagneticField, Optimizable):
     """Magnetic field scaled by a scalar value.
 
diff --git a/docs/api_fields.rst b/docs/api_fields.rst
index 63c0648f8a..32b5cc572b 100644
--- a/docs/api_fields.rst
+++ b/docs/api_fields.rst
@@ -23,6 +23,7 @@ a surface and computes the normal field strength on that surface.
 
     desc.magnetic_fields.SplineMagneticField
     desc.magnetic_fields.DommaschkPotentialField
+    desc.magnetic_fields.MagneticFieldFromUser
     desc.magnetic_fields.ScalarPotentialField
     desc.magnetic_fields.ToroidalMagneticField
     desc.magnetic_fields.VerticalMagneticField
diff --git a/tests/test_magnetic_fields.py b/tests/test_magnetic_fields.py
index ae5b51c35f..9e4103f0dc 100644
--- a/tests/test_magnetic_fields.py
+++ b/tests/test_magnetic_fields.py
@@ -15,6 +15,7 @@
     CurrentPotentialField,
     DommaschkPotentialField,
     FourierCurrentPotentialField,
+    MagneticFieldFromUser,
     OmnigenousField,
     PoloidalMagneticField,
     ScalarPotentialField,
@@ -64,6 +65,28 @@ def test_basic_fields(self):
             (tfield + vfield - pfield)([1, 0, 0.1]), [[0.4, 2, 1]]
         )
 
+    @pytest.mark.unit
+    def test_field_from_user(self):
+        """Test for MagneticFieldFromUser."""
+        tfield = ToroidalMagneticField(2, 1)
+
+        def fun(coords, params):
+            R0, B0 = params
+            coords = jnp.atleast_2d(jnp.asarray(coords))
+            bp = B0 * R0 / coords[:, 0]
+            brz = jnp.zeros_like(bp)
+            B = jnp.array([brz, bp, brz]).T
+            return B
+
+        ufield = MagneticFieldFromUser(fun, [tfield.R0, tfield.B0])
+        np.testing.assert_allclose(
+            tfield([1, 0, 0]), ufield([1, 0, 0], params=[tfield.R0, tfield.B0])
+        )
+        np.testing.assert_allclose(
+            tfield([1, 1, 0], basis="xyz"),
+            ufield([1, 1, 0], params=[tfield.R0, tfield.B0], basis="xyz"),
+        )
+
     @pytest.mark.unit
     def test_combined_fields(self):
         """Tests for sum/scaled fields."""