diff --git a/qiskit/circuit/library/standard_gates/x.py b/qiskit/circuit/library/standard_gates/x.py
index 7195df90dc98..ddcf93d68b33 100644
--- a/qiskit/circuit/library/standard_gates/x.py
+++ b/qiskit/circuit/library/standard_gates/x.py
@@ -21,6 +21,7 @@
 from qiskit.circuit._utils import _ctrl_state_to_int, with_gate_array, with_controlled_gate_array
 
 _X_ARRAY = [[0, 1], [1, 0]]
+_SX_ARRAY = [[0.5 + 0.5j, 0.5 - 0.5j], [0.5 - 0.5j, 0.5 + 0.5j]]
 
 
 @with_gate_array(_X_ARRAY)
@@ -562,6 +563,7 @@ def __eq__(self, other):
         return isinstance(other, RCCXGate)
 
 
+@with_controlled_gate_array(_SX_ARRAY, num_ctrl_qubits=3, cached_states=(7,))
 class C3SXGate(SingletonControlledGate):
     """The 3-qubit controlled sqrt-X gate.
 
diff --git a/releasenotes/notes/fix-c3sx-gate-matrix-050cf9f9ac3b2b82.yaml b/releasenotes/notes/fix-c3sx-gate-matrix-050cf9f9ac3b2b82.yaml
new file mode 100644
index 000000000000..accd20de4ae5
--- /dev/null
+++ b/releasenotes/notes/fix-c3sx-gate-matrix-050cf9f9ac3b2b82.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a missing decorator in :class:`.C3SXGate` that made it fail if :meth:`.Gate.to_matrix` was called.
+    The gate matrix is now return as expected.
diff --git a/test/python/circuit/test_controlled_gate.py b/test/python/circuit/test_controlled_gate.py
index ced7229415ea..580bd9d4df48 100644
--- a/test/python/circuit/test_controlled_gate.py
+++ b/test/python/circuit/test_controlled_gate.py
@@ -230,6 +230,12 @@ def test_special_cases_equivalent_to_controlled_base_gate(self):
                 # Ensure that both the array form (if the gate overrides `__array__`) and the
                 # circuit-definition form are tested.
                 self.assertTrue(Operator(special_case_gate).equiv(naive_operator))
+                if not isinstance(special_case_gate, (MCXGate, MCPhaseGate, MCU1Gate)):
+                    # Ensure that the to_matrix method yields the same result
+                    np.testing.assert_allclose(
+                        special_case_gate.to_matrix(), naive_operator.to_matrix(), atol=1e-8
+                    )
+
                 if not isinstance(special_case_gate, CXGate):
                     # CX is treated like a primitive within Terra, and doesn't have a definition.
                     self.assertTrue(Operator(special_case_gate.definition).equiv(naive_operator))