diff --git a/opensquirrel/circuit_builder.py b/opensquirrel/circuit_builder.py
index 47272c18..0bf18ee3 100644
--- a/opensquirrel/circuit_builder.py
+++ b/opensquirrel/circuit_builder.py
@@ -80,6 +80,13 @@ def __getattr__(self, attr: str) -> Any:
         # Default behaviour
         return self.__getattribute__(attr)
 
+    def __contains__(self, item: object) -> bool:
+        if isinstance(item, QubitRegister):
+            return item.name in self.register_manager._qubit_registry  # noqa: SLF001
+        if isinstance(item, BitRegister):
+            return item.name in self.register_manager._bit_registry  # noqa: SLF001
+        return False
+
     def add_register(self, register: QubitRegister | BitRegister) -> None:
         """Add a (qu)bit register to the circuit builder.
 
diff --git a/tests/test_circuit_builder.py b/tests/test_circuit_builder.py
index 3579267f..98ed986a 100644
--- a/tests/test_circuit_builder.py
+++ b/tests/test_circuit_builder.py
@@ -629,3 +629,32 @@ def test_add_instruction_non_unitaries(self, instruction: Instruction, expected:
         circuit = builder.to_circuit()
 
         assert circuit.ir.statements == [expected]
+
+
+@pytest.fixture
+def qubit_register1() -> QubitRegister:
+    return QubitRegister(4, "q0")
+
+
+@pytest.fixture
+def qubit_register2() -> QubitRegister:
+    return QubitRegister(8, "q1")
+
+
+@pytest.fixture
+def bit_register() -> BitRegister:
+    return BitRegister(4, "b0")
+
+
+def test_qubit_register_in_builder(qubit_register1: QubitRegister, qubit_register2: QubitRegister) -> None:
+    builder = CircuitBuilder()
+    builder.add_register(qubit_register1)
+    assert qubit_register1 in builder
+    assert qubit_register2 not in builder
+
+
+def test_bit_register_in_builder(bit_register: BitRegister) -> None:
+    builder = CircuitBuilder()
+    builder.add_register(bit_register)
+    assert bit_register in builder
+    assert BitRegister(8, "b1") not in builder