diff --git a/opensquirrel/circuit_builder.py b/opensquirrel/circuit_builder.py
index 47272c18..c7f32b61 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
+        if isinstance(item, BitRegister):
+            return item.name in self.register_manager.bit_registry
+        return False
+
     def add_register(self, register: QubitRegister | BitRegister) -> None:
         """Add a (qu)bit register to the circuit builder.
 
diff --git a/opensquirrel/register_manager.py b/opensquirrel/register_manager.py
index fc1bd9a5..591e7e21 100644
--- a/opensquirrel/register_manager.py
+++ b/opensquirrel/register_manager.py
@@ -105,6 +105,14 @@ def __init__(self, qubit_registry: QubitRegistry, bit_registry: BitRegistry) ->
             BitRegister(0) if not bit_registry else (RegisterManager.generate_virtual_register(bit_registry))
         )
 
+    @property
+    def qubit_registry(self) -> QubitRegistry:
+        return self._qubit_registry
+
+    @property
+    def bit_registry(self) -> BitRegistry:
+        return self._bit_registry
+
     @property
     def qubit_register_size(self) -> int:
         return self._virtual_qubit_register.size
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