diff --git a/cirq-core/cirq/ops/fsim_gate.py b/cirq-core/cirq/ops/fsim_gate.py
index 99598101661..561fd38f2ed 100644
--- a/cirq-core/cirq/ops/fsim_gate.py
+++ b/cirq-core/cirq/ops/fsim_gate.py
@@ -23,18 +23,18 @@
 
 import cmath
 import math
-from typing import AbstractSet, Any, Dict, Optional, Tuple, Union
+from typing import AbstractSet, Any, Dict, Optional, Tuple
 
 import numpy as np
 import sympy
 
 import cirq
 from cirq import protocols, value
-from cirq._compat import proper_repr
+from cirq._compat import deprecated, proper_repr
 from cirq.ops import gate_features, raw_types
 
 
-def _canonicalize(value: Union[float, sympy.Basic]) -> Union[float, sympy.Basic]:
+def _canonicalize(value: 'cirq.TParamVal') -> 'cirq.TParamVal':
     """Assumes value is 2π-periodic and shifts it into [-π, π)."""
     if protocols.is_parameterized(value):
         return value
@@ -42,12 +42,12 @@ def _canonicalize(value: Union[float, sympy.Basic]) -> Union[float, sympy.Basic]
     return value - period * np.floor((value + np.pi) / period)
 
 
-def _zero_mod_pi(param: Union[float, sympy.Basic]) -> bool:
+def _zero_mod_pi(param: 'cirq.TParamVal') -> bool:
     """Returns True iff param, assumed to be in [-pi, pi), is 0 (mod pi)."""
     return param in (0.0, -np.pi, -sympy.pi)
 
 
-def _half_pi_mod_pi(param: Union[float, sympy.Basic]) -> bool:
+def _half_pi_mod_pi(param: 'cirq.TParamVal') -> bool:
     """Returns True iff param, assumed to be in [-pi, pi), is pi/2 (mod pi)."""
     return param in (-np.pi / 2, np.pi / 2, -sympy.pi / 2, sympy.pi / 2)
 
@@ -78,7 +78,7 @@ class FSimGate(gate_features.InterchangeableQubitsGate, raw_types.Gate):
         FSimGate(θ, φ) = ISWAP**(-2θ/π) CZPowGate(exponent=-φ/π)
     """
 
-    def __init__(self, theta: float, phi: float) -> None:
+    def __init__(self, theta: 'cirq.TParamVal', phi: 'cirq.TParamVal') -> None:
         """Inits FSimGate.
 
         Args:
@@ -90,8 +90,32 @@ def __init__(self, theta: float, phi: float) -> None:
                 ``|11⟩`` state is phased. Note: uses opposite sign convention to
                 the CZPowGate. Maximum strength (full cz) is at pi.
         """
-        self.theta = _canonicalize(theta)
-        self.phi = _canonicalize(phi)
+        self._theta = _canonicalize(theta)
+        self._phi = _canonicalize(phi)
+
+    @property
+    def theta(self) -> 'cirq.TParamVal':
+        return self._theta
+
+    @theta.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def theta(self, theta: 'cirq.TParamVal'):
+        self._theta = theta
+
+    @property
+    def phi(self) -> 'cirq.TParamVal':
+        return self._phi
+
+    @phi.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def phi(self, phi: 'cirq.TParamVal'):
+        self._phi = phi
 
     def _num_qubits_(self) -> int:
         return 2
@@ -249,11 +273,11 @@ class PhasedFSimGate(gate_features.InterchangeableQubitsGate, raw_types.Gate):
 
     def __init__(
         self,
-        theta: Union[float, sympy.Basic],
-        zeta: Union[float, sympy.Basic] = 0.0,
-        chi: Union[float, sympy.Basic] = 0.0,
-        gamma: Union[float, sympy.Basic] = 0.0,
-        phi: Union[float, sympy.Basic] = 0.0,
+        theta: 'cirq.TParamVal',
+        zeta: 'cirq.TParamVal' = 0.0,
+        chi: 'cirq.TParamVal' = 0.0,
+        gamma: 'cirq.TParamVal' = 0.0,
+        phi: 'cirq.TParamVal' = 0.0,
     ) -> None:
         """Inits PhasedFSimGate.
 
@@ -269,18 +293,78 @@ def __init__(
             phi: Controlled phase angle, in radians. See class docstring
                 above for details.
         """
-        self.theta = _canonicalize(theta)
-        self.zeta = _canonicalize(zeta)
-        self.chi = _canonicalize(chi)
-        self.gamma = _canonicalize(gamma)
-        self.phi = _canonicalize(phi)
+        self._theta = _canonicalize(theta)
+        self._zeta = _canonicalize(zeta)
+        self._chi = _canonicalize(chi)
+        self._gamma = _canonicalize(gamma)
+        self._phi = _canonicalize(phi)
+
+    @property
+    def theta(self) -> 'cirq.TParamVal':
+        return self._theta
+
+    @theta.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def theta(self, theta: 'cirq.TParamVal'):
+        self._theta = theta
+
+    @property
+    def zeta(self) -> 'cirq.TParamVal':
+        return self._zeta
+
+    @zeta.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def zeta(self, zeta: 'cirq.TParamVal'):
+        self._zeta = zeta
+
+    @property
+    def chi(self) -> 'cirq.TParamVal':
+        return self._chi
+
+    @chi.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def chi(self, chi: 'cirq.TParamVal'):
+        self._chi = chi
+
+    @property
+    def gamma(self) -> 'cirq.TParamVal':
+        return self._gamma
+
+    @gamma.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def gamma(self, gamma: 'cirq.TParamVal'):
+        self._gamma = gamma
+
+    @property
+    def phi(self) -> 'cirq.TParamVal':
+        return self._phi
+
+    @phi.setter  # type: ignore
+    @deprecated(
+        deadline="v0.15",
+        fix="The mutators of this class are deprecated, instantiate a new object instead.",
+    )
+    def phi(self, phi: 'cirq.TParamVal'):
+        self._phi = phi
 
     @staticmethod
     def from_fsim_rz(
-        theta: Union[float, sympy.Basic],
-        phi: Union[float, sympy.Basic],
-        rz_angles_before: Tuple[Union[float, sympy.Basic], Union[float, sympy.Basic]],
-        rz_angles_after: Tuple[Union[float, sympy.Basic], Union[float, sympy.Basic]],
+        theta: 'cirq.TParamVal',
+        phi: 'cirq.TParamVal',
+        rz_angles_before: Tuple['cirq.TParamVal', 'cirq.TParamVal'],
+        rz_angles_after: Tuple['cirq.TParamVal', 'cirq.TParamVal'],
     ) -> 'PhasedFSimGate':
         """Creates PhasedFSimGate using an alternate parametrization.
 
@@ -302,14 +386,14 @@ def from_fsim_rz(
         return PhasedFSimGate(theta, zeta, chi, gamma, phi)
 
     @property
-    def rz_angles_before(self) -> Tuple[Union[float, sympy.Basic], Union[float, sympy.Basic]]:
+    def rz_angles_before(self) -> Tuple['cirq.TParamVal', 'cirq.TParamVal']:
         """Returns 2-tuple of phase angles applied to qubits before FSimGate."""
         b0 = (-self.gamma + self.zeta + self.chi) / 2.0
         b1 = (-self.gamma - self.zeta - self.chi) / 2.0
         return b0, b1
 
     @property
-    def rz_angles_after(self) -> Tuple[Union[float, sympy.Basic], Union[float, sympy.Basic]]:
+    def rz_angles_after(self) -> Tuple['cirq.TParamVal', 'cirq.TParamVal']:
         """Returns 2-tuple of phase angles applied to qubits after FSimGate."""
         a0 = (-self.gamma + self.zeta - self.chi) / 2.0
         a1 = (-self.gamma - self.zeta + self.chi) / 2.0
@@ -413,7 +497,7 @@ def _decompose_(self, qubits) -> 'cirq.OP_TREE':
         makes the top left element of the matrix equal to 1.
         """
 
-        def to_exponent(angle_rads: Union[float, sympy.Basic]) -> Union[float, sympy.Basic]:
+        def to_exponent(angle_rads: 'cirq.TParamVal') -> 'cirq.TParamVal':
             """Divides angle_rads by symbolic or numerical pi."""
             pi = sympy.pi if protocols.is_parameterized(angle_rads) else np.pi
             return angle_rads / pi
diff --git a/cirq-core/cirq/ops/fsim_gate_test.py b/cirq-core/cirq/ops/fsim_gate_test.py
index b9e41b1a81b..2b0bd2a7f9c 100644
--- a/cirq-core/cirq/ops/fsim_gate_test.py
+++ b/cirq-core/cirq/ops/fsim_gate_test.py
@@ -804,3 +804,39 @@ def test_phased_fsim_json_dict():
         'gamma': 0.78,
         'phi': 0.9,
     }
+
+
+def test_setters_deprecated():
+    gate = cirq.FSimGate(0.1, 0.1)
+    assert gate.theta == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.theta = 0.2
+        assert gate.theta == 0.2
+    assert gate.phi == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.phi = 0.2
+        assert gate.phi == 0.2
+
+
+def test_phased_setters_deprecated():
+    gate = cirq.PhasedFSimGate(0.1, 0.1, 0.1, 0.1, 0.1)
+    assert gate.theta == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.theta = 0.2
+        assert gate.theta == 0.2
+    assert gate.zeta == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.zeta = 0.2
+        assert gate.zeta == 0.2
+    assert gate.chi == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.chi = 0.2
+        assert gate.chi == 0.2
+    assert gate.gamma == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.gamma = 0.2
+        assert gate.gamma == 0.2
+    assert gate.phi == 0.1
+    with cirq.testing.assert_deprecated('mutators', deadline='v0.15'):
+        gate.phi = 0.2
+        assert gate.phi == 0.2