diff --git a/pytket/docs/changelog.rst b/pytket/docs/changelog.rst
index 96ca9dc38e..2e215cb5f1 100644
--- a/pytket/docs/changelog.rst
+++ b/pytket/docs/changelog.rst
@@ -8,6 +8,10 @@ Features:
 
 * Add `RemovePhaseOps` pass.
 
+Fixes:
+
+* Fix issue with custom gates in qasm to circuit conversion.
+
 1.38.0 (January 2025)
 ---------------------
 
diff --git a/pytket/pytket/qasm/qasm.py b/pytket/pytket/qasm/qasm.py
index 605689dd1c..a0f20e41eb 100644
--- a/pytket/pytket/qasm/qasm.py
+++ b/pytket/pytket/qasm/qasm.py
@@ -183,6 +183,16 @@ class QASMUnsupportedError(Exception):
     "fsim": OpType.FSim,
 }
 
+N_PARAMS_EXTRA_COMMANDS = {
+    OpType.TK2: 3,
+    OpType.ISWAP: 1,
+    OpType.PhasedISWAP: 2,
+    OpType.YYPhase: 1,
+    OpType.XXPhase3: 1,
+    OpType.ESWAP: 1,
+    OpType.FSim: 2,
+}
+
 _tk_to_qasm_noparams = dict((item[1], item[0]) for item in NOPARAM_COMMANDS.items())
 _tk_to_qasm_noparams[OpType.CX] = "cx"  # prefer "cx" to "CX"
 _tk_to_qasm_params = dict((item[1], item[0]) for item in PARAM_COMMANDS.items())
@@ -859,9 +869,7 @@ def transform(self, tree: Tree) -> dict[str, Any]:
 
     def gdef(self, tree: list) -> None:
         child_iter = iter(tree)
-
         gate = next(child_iter).value
-
         next_tree = next(child_iter)
         symbols, args = [], []
         if isinstance(next_tree, ParsMap):
@@ -882,6 +890,9 @@ def gdef(self, tree: list) -> None:
         # check to see whether gate definition was generated by pytket converter
         # if true, add op as pytket Op
         existing_op: bool = False
+        # NOPARAM_EXTRA_COMMANDS and PARAM_EXTRA_COMMANDS are
+        # gates that aren't in the standard qasm spec but in the standard TKET
+        # optypes
         if gate in NOPARAM_EXTRA_COMMANDS:
             qubit_args = [
                 Qubit(gate + "q" + str(index), 0) for index in list(range(len(args)))
@@ -894,25 +905,33 @@ def gdef(self, tree: list) -> None:
             ) == circuit_to_qasm_str(gate_circ, maxwidth=self.maxwidth):
                 existing_op = True
         elif gate in PARAM_EXTRA_COMMANDS:
-            qubit_args = [
-                Qubit(gate + "q" + str(index), 0) for index in list(range(len(args)))
-            ]
-            comparison_circ = _get_gate_circuit(
-                PARAM_EXTRA_COMMANDS[gate],
-                qubit_args,
-                [Symbol("param" + str(index) + "/pi") for index in range(len(symbols))],
-            )
-            # checks that each command has same string
-            existing_op = all(
-                str(g) == str(c)
-                for g, c in zip(
-                    gate_circ.get_commands(), comparison_circ.get_commands()
+            optype = PARAM_EXTRA_COMMANDS[gate]
+            # we check this here, as _get_gate_circuit will find issue if it isn't true
+            # the later existing_op=all check will make sure it's the same circuit later
+            if len(symbols) != N_PARAMS_EXTRA_COMMANDS[optype]:
+                existing_op = False
+            else:
+                qubit_args = [
+                    Qubit(gate + "q" + str(index), 0) for index in range(len(args))
+                ]
+                comparison_circ = _get_gate_circuit(
+                    optype,
+                    qubit_args,
+                    [
+                        Symbol("param" + str(index) + "/pi")
+                        for index in range(len(symbols))
+                    ],
+                )
+                # checks that each command has same string
+                existing_op = all(
+                    str(g) == str(c)
+                    for g, c in zip(
+                        gate_circ.get_commands(), comparison_circ.get_commands()
+                    )
                 )
-            )
         if not existing_op:
             gate_circ.symbol_substitution(symbol_map)
             gate_circ.rename_units(cast(dict[UnitID, UnitID], rename_map))
-
             self.gate_dict[gate] = {
                 "definition": gate_circ.to_dict(),
                 "args": symbols,
diff --git a/pytket/tests/qasm_test.py b/pytket/tests/qasm_test.py
index a3824c03fe..88f70c6d20 100644
--- a/pytket/tests/qasm_test.py
+++ b/pytket/tests/qasm_test.py
@@ -1209,6 +1209,23 @@ def test_multibitop() -> None:
     )
 
 
+def test_existing_name_conversion() -> None:
+    # https://github.com/CQCL/tket/issues/1605
+    assert (
+        circuit_from_qasm_str(
+            """OPENQASM 2.0;
+include "qelib1.inc";
+gate iswap q0,q1 { s q0; s q1; h q0; cx q0,q1; cx q1,q0; h q1; }
+qreg qr[3];
+creg cr[3];
+iswap qr[1],qr[2];"""
+        )
+        .get_commands()[0]
+        .op.type
+        == OpType.CustomGate
+    )
+
+
 if __name__ == "__main__":
     test_qasm_correct()
     test_qasm_qubit()
@@ -1247,3 +1264,4 @@ def test_multibitop() -> None:
     test_classical_expbox_arg_order(True)
     test_classical_expbox_arg_order(False)
     test_register_name_check()
+    test_existing_name_conversion()