diff --git a/janus_core/calculations/geom_opt.py b/janus_core/calculations/geom_opt.py
index 49a3e147..9b370780 100644
--- a/janus_core/calculations/geom_opt.py
+++ b/janus_core/calculations/geom_opt.py
@@ -3,11 +3,12 @@
 from __future__ import annotations
 
 from collections.abc import Callable
+import inspect
 from pathlib import Path
 from typing import Any
 import warnings
 
-from ase import Atoms, filters, units
+from ase import Atoms, constraints, filters, units
 from ase.filters import FrechetCellFilter
 from ase.io import read
 import ase.optimize
@@ -76,6 +77,10 @@ class GeomOpt(BaseCalculation):
         Default is `FrechetCellFilter`.
     filter_kwargs
         Keyword arguments to pass to filter_class. Default is {}.
+    constraint_class
+        Constraint class, or name of class from ase.constraints. Default is None.
+    constraint_kwargs
+        Keyword arguments to pass to constraint_class. Default is {}.
     optimizer
         Optimization function, or name of function from ase.optimize. Default is
         `LBFGS`.
@@ -113,6 +118,8 @@ def __init__(
         angle_tolerance: float = -1.0,
         filter_class: Callable | str | None = FrechetCellFilter,
         filter_kwargs: dict[str, Any] | None = None,
+        constraint_class: type | str | None = None,
+        constraint_kwargs: dict[str, Any] | None = None,
         optimizer: Callable | str = LBFGS,
         opt_kwargs: ASEOptArgs | None = None,
         write_results: bool = False,
@@ -167,6 +174,11 @@ def __init__(
             Default is `FrechetCellFilter`.
         filter_kwargs
             Keyword arguments to pass to filter_class. Default is {}.
+        constraint_class
+            Constraint class, or name of class from ase.constraints. Default is
+            None.
+        constraint_kwargs
+            Keyword arguments to construct constraint_class. Default is {}.
         optimizer
             Optimization function, or name of function from ase.optimize. Default is
             `LBFGS`.
@@ -184,9 +196,21 @@ def __init__(
             "filename" keyword is inferred from `file_prefix` if not given.
             Default is {}.
         """
-        read_kwargs, filter_kwargs, opt_kwargs, write_kwargs, traj_kwargs = (
+        (
+            read_kwargs,
+            constraint_kwargs,
+            filter_kwargs,
+            opt_kwargs,
+            write_kwargs,
+            traj_kwargs,
+        ) = list(
             none_to_dict(
-                read_kwargs, filter_kwargs, opt_kwargs, write_kwargs, traj_kwargs
+                read_kwargs,
+                constraint_kwargs,
+                filter_kwargs,
+                opt_kwargs,
+                write_kwargs,
+                traj_kwargs,
             )
         )
 
@@ -197,6 +221,8 @@ def __init__(
         self.angle_tolerance = angle_tolerance
         self.filter_class = filter_class
         self.filter_kwargs = filter_kwargs
+        self.constraint_class = constraint_class
+        self.constraint_kwargs = constraint_kwargs
         self.optimizer = optimizer
         self.opt_kwargs = opt_kwargs
         self.write_results = write_results
@@ -277,12 +303,29 @@ def output_files(self) -> None:
             "trajectory": self.traj_kwargs.get("filename"),
         }
 
+    def _set_mandatory_constraint_kwargs(self) -> None:
+        """
+        Inspect constraint class for mandatory arguments.
+
+        For now we are just looking for the "atoms" parameter of FixSymmetry
+        """
+        parameters = inspect.signature(self.constraint_class.__init__).parameters
+        if "atoms" in parameters:
+            self.constraint_kwargs["atoms"] = self.struct
+
     def set_optimizer(self) -> None:
         """Set optimizer for geometry optimization."""
         self._set_functions()
         if self.logger:
             self.logger.info("Using optimizer: %s", self.optimizer.__name__)
 
+        if self.constraint_class is not None:
+            self._set_mandatory_constraint_kwargs()
+            self.struct.set_constraint(self.constraint_class(**self.constraint_kwargs))
+
+            if self.logger:
+                self.logger.info("Using constraint: %s", self.constraint_class.__name__)
+
         if self.filter_class is not None:
             if "scalar_pressure" in self.filter_kwargs:
                 self.filter_kwargs["scalar_pressure"] *= units.GPa
@@ -308,13 +351,21 @@ def set_optimizer(self) -> None:
             self.dyn = self.optimizer(self.struct, **self.opt_kwargs)
 
     def _set_functions(self) -> None:
-        """Set optimizer and filter."""
+        """Set optimizer, constraint and filter functions."""
         if isinstance(self.optimizer, str):
             try:
                 self.optimizer = getattr(ase.optimize, self.optimizer)
             except AttributeError as e:
                 raise AttributeError(f"No such optimizer: {self.optimizer}") from e
 
+        if self.constraint_class is not None and isinstance(self.constraint_class, str):
+            try:
+                self.constraint_class = getattr(constraints, self.constraint_class)
+            except AttributeError as e:
+                raise AttributeError(
+                    f"No such constraint: {self.constraint_class}"
+                ) from e
+
         if self.filter_class is not None and isinstance(self.filter_class, str):
             try:
                 self.filter_class = getattr(filters, self.filter_class)
diff --git a/janus_core/cli/geomopt.py b/janus_core/cli/geomopt.py
index 78ad3350..12c11d3f 100644
--- a/janus_core/cli/geomopt.py
+++ b/janus_core/cli/geomopt.py
@@ -124,6 +124,14 @@ def geomopt(
             rich_help_panel="Calculation",
         ),
     ] = None,
+    constraint_class: Annotated[
+        str,
+        Option(
+            "--constraint",
+            help="Name of ASE constraint to attach to atoms.",
+            rich_help_panel="Calculation",
+        ),
+    ] = None,
     pressure: Annotated[
         float,
         Option(
@@ -205,6 +213,10 @@ def geomopt(
     filter_class
         Name of filter from ase.filters to wrap around atoms. If using
         --opt-cell-lengths or --opt-cell-fully, defaults to `FrechetCellFilter`.
+    constraint_class
+        Name of constraint class from ase.constraints, to apply constraints
+        to atoms. Parameters should be included as a "constraint_kwargs" dict
+        within "minimize_kwargs". Default is None.
     pressure
         Scalar pressure when optimizing cell geometry, in GPa. Passed to the filter
         function if either `opt_cell_lengths` or `opt_cell_fully` is True. Default is
@@ -283,6 +295,7 @@ def geomopt(
     # Check optimized structure path not duplicated
     if "filename" in write_kwargs:
         raise ValueError("'filename' must be passed through the --out option")
+
     if out:
         write_kwargs["filename"] = out
 
@@ -323,6 +336,7 @@ def geomopt(
         "symmetrize": symmetrize,
         "symmetry_tolerance": symmetry_tolerance,
         "file_prefix": file_prefix,
+        "constraint_class": constraint_class,
         **opt_cell_fully_dict,
         **minimize_kwargs,
         "write_results": True,
diff --git a/tests/test_geom_opt.py b/tests/test_geom_opt.py
index 67fa314a..8d3d6cb4 100644
--- a/tests/test_geom_opt.py
+++ b/tests/test_geom_opt.py
@@ -6,6 +6,7 @@
 
 from ase.filters import FrechetCellFilter, UnitCellFilter
 from ase.io import read
+from numpy.testing import assert_allclose
 import pytest
 
 from janus_core.calculations.geom_opt import GeomOpt
@@ -52,6 +53,28 @@ def test_optimize(arch, struct, expected, kwargs):
     )
 
 
+def test_constrained_optimize():
+    """Test optimizing geometry using MACE with ASE constraint."""
+    single_point = SinglePoint(
+        struct=DATA_PATH / "H2O.cif",
+        arch="mace",
+        model=MODEL_PATH,
+    )
+
+    initial_positions = single_point.struct.positions.copy()
+
+    optimizer = GeomOpt(
+        single_point.struct,
+        filter_class=None,  # No volume opt
+        constraint_class="FixAtoms",
+        constraint_kwargs={"indices": [2]},
+    )
+    optimizer.run()
+
+    assert_allclose(initial_positions[2], optimizer.struct.positions[2])
+    assert (initial_positions[:2] - optimizer.struct.positions[:2]).any()
+
+
 def test_saving_struct(tmp_path):
     """Test saving optimized structure."""
     results_path = tmp_path / "NaCl.extxyz"
diff --git a/tests/test_geomopt_cli.py b/tests/test_geomopt_cli.py
index 1ef21e4b..494b6754 100644
--- a/tests/test_geomopt_cli.py
+++ b/tests/test_geomopt_cli.py
@@ -603,6 +603,39 @@ def test_filter_str_error(tmp_path):
     assert isinstance(result.exception, ValueError)
 
 
+def test_constraint(tmp_path):
+    """Test setting constraint."""
+    results_path = tmp_path / "H2O-opt.extxyz"
+    log_path = tmp_path / "test.log"
+    summary_path = tmp_path / "summary.yml"
+
+    result = runner.invoke(
+        app,
+        [
+            "geomopt",
+            "--struct",
+            DATA_PATH / "H2O.cif",
+            "--arch",
+            "mace_mp",
+            "--out",
+            results_path,
+            "--constraint",
+            "FixAtoms",
+            "--minimize-kwargs",
+            "{'constraint_kwargs': {'indices': [2]}}",
+            "--log",
+            log_path,
+            "--summary",
+            summary_path,
+        ],
+    )
+    assert result.exit_code == 0
+    assert_log_contains(
+        log_path,
+        includes=["Starting geometry optimization", "Using constraint: FixAtoms"],
+    )
+
+
 @pytest.mark.parametrize("read_kwargs", ["{'index': 1}", "{}"])
 def test_valid_traj_input(read_kwargs, tmp_path):
     """Test valid trajectory input structure handled."""