diff --git a/CHANGELOG.md b/CHANGELOG.md
index 762a23d019..12a7d921d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 
 ## Bug Fixes
 
+- Added `_from_json()` functionality to `Sign` which was erroneously omitted previously. ([#4517](https://github.com/pybamm-team/PyBaMM/pull/4517))
 - Fixed bug where IDAKLU solver failed when `output variables` were specified and an extrapolation event is present. ([#4440](https://github.com/pybamm-team/PyBaMM/pull/4440))
 
 ## Breaking changes
diff --git a/src/pybamm/expression_tree/unary_operators.py b/src/pybamm/expression_tree/unary_operators.py
index ace1cd9942..aa90fd6f4c 100644
--- a/src/pybamm/expression_tree/unary_operators.py
+++ b/src/pybamm/expression_tree/unary_operators.py
@@ -212,7 +212,8 @@ def __init__(self, child):
 
     @classmethod
     def _from_json(cls, snippet: dict):
-        raise NotImplementedError()
+        """See :meth:`pybamm.UnaryOperator._from_json()`."""
+        return cls(snippet["children"][0])
 
     def diff(self, variable):
         """See :meth:`pybamm.Symbol.diff()`."""
diff --git a/tests/unit/test_expression_tree/test_unary_operators.py b/tests/unit/test_expression_tree/test_unary_operators.py
index 0dbafa38c5..d7544763fe 100644
--- a/tests/unit/test_expression_tree/test_unary_operators.py
+++ b/tests/unit/test_expression_tree/test_unary_operators.py
@@ -138,9 +138,20 @@ def test_sign(self):
         )
 
         # Test from_json
-        with pytest.raises(NotImplementedError):
-            # signs are always scalar/array types in a discretised model
-            pybamm.Sign._from_json({})
+        c = pybamm.Multiplication(pybamm.Variable("a"), pybamm.Scalar(3))
+        sign_json = {
+            "name": "sign",
+            "id": 5341515228900508018,
+            "domains": {
+                "primary": [],
+                "secondary": [],
+                "tertiary": [],
+                "quaternary": [],
+            },
+            "children": [c],
+        }
+
+        assert pybamm.sign(c) == pybamm.Sign._from_json(sign_json)
 
     def test_floor(self, mocker):
         a = pybamm.Symbol("a")