@@ -1596,6 +1596,12 @@
 * Various decomposition rules are updated so that they accept positionally passed arguments.
   [(#10088)](https://github.com/PennyLaneAI/pennylane/pull/10088)
 
+* Fixed :class:`~pennylane.ops.op_math.Pow` and ``Pow2`` returning the complex conjugate of the
+  correct eigenvalues for fractional powers of operators with negative eigenvalues on Python 3.14.
+  Python 3.14 implements C99 mixed-mode arithmetic, so multiplying a negative real by ``1 + 0j`` now
+  yields a negative zero imaginary part, which selects the wrong branch of the power function.
+  [(#10139)](https://github.com/PennyLaneAI/pennylane/pull/10139)
+
 <h3>Contributors ✍️</h3>
 
 This release contains contributions from (in alphabetical order):
 
@@ -336,8 +336,11 @@ def diagonalizing_gates(self):
         return self.base.diagonalizing_gates()
 
     def eigvals(self):
+        # Cast rather than multiply by ``1 + 0j``: since Python 3.14 implements C99 mixed-mode
+        # arithmetic, ``(1 + 0j) * -1.0`` is ``-1 - 0j``, and the negative zero imaginary part
+        # puts negative eigenvalues on the wrong side of the branch cut of ``**``.
         base_eigvals = self.base.eigvals()
-        return [((1 + 0j) * value) ** self.z for value in base_eigvals]
+        return math.cast(base_eigvals, "complex128") ** self.z
 
     # pylint: disable=arguments-renamed, invalid-overridden-method
     @property
 
@@ -241,8 +241,11 @@ def compute_diagonalizing_gates(base, z):  # pylint: disable=unused-argument
 
     @override
     def eigvals(self):
+        # Cast rather than multiply by ``1 + 0j``: since Python 3.14 implements C99 mixed-mode
+        # arithmetic, ``(1 + 0j) * -1.0`` is ``-1 - 0j``, and the negative zero imaginary part
+        # puts negative eigenvalues on the wrong side of the branch cut of ``**``.
         base_eigvals = self.base.eigvals()
-        return [((1 + 0j) * value) ** self.z for value in base_eigvals]
+        return math.cast(base_eigvals, "complex128") ** self.z
 
     # pylint: disable=arguments-renamed, invalid-overridden-method
     @property
 
@@ -356,6 +356,8 @@ def test_eigvals_fractional_power_negative_eigenvalue_2(z, power_method):
     expected_eigvals = np.array([1.0**z, (-1.0 + 0j) ** z])
 
     assert np.allclose(eigvals, expected_eigvals)
+    # the eigenvalues must sit on the same branch of ``**`` as the matrix
+    assert np.allclose(eigvals, np.diag(qp.matrix(op)))
 
 
 # pylint: disable-next=too-few-public-methods
 
@@ -446,6 +446,8 @@ def test_eigvals_fractional_power_negative_eigenvalue(self, z, power_method):
         expected_eigvals = np.array([1.0**z, (-1.0 + 0j) ** z])
 
         assert np.allclose(eigvals, expected_eigvals)
+        # the eigenvalues must sit on the same branch of ``**`` as the matrix
+        assert np.allclose(eigvals, np.diag(qp.matrix(op)))
 
 
 class TestSimplify: