diff --git a/Mathlib/LinearAlgebra/Matrix/ConjTranspose.lean b/Mathlib/LinearAlgebra/Matrix/ConjTranspose.lean
index 5c730990d2ca34..6acd1c31f97c1d 100644
--- a/Mathlib/LinearAlgebra/Matrix/ConjTranspose.lean
+++ b/Mathlib/LinearAlgebra/Matrix/ConjTranspose.lean
@@ -424,14 +424,11 @@ instance [Star α] [Star β] [SMul α β] [StarModule α β] : StarModule α (Ma
   star_smul := conjTranspose_smul
 
 /-- When `α` is a \*-(semi)ring, `Matrix.star` is also a \*-(semi)ring. -/
-instance [Fintype n] [NonUnitalSemiring α] [StarRing α] : StarRing (Matrix n n α) where
+instance [Fintype n] [NonUnitalNonAssocSemiring α] [StarRing α] : StarRing (Matrix n n α) where
   star_add := conjTranspose_add
   star_mul := conjTranspose_mul
 
-/-- A version of `star_mul` for `*` instead of `*`. -/
-theorem star_mul [Fintype n] [NonUnitalNonAssocSemiring α] [StarRing α] (M N : Matrix n n α) :
-    star (M * N) = star N * star M :=
-  conjTranspose_mul _ _
+@[deprecated (since := "2026-04-20")] protected alias star_mul := StarMul.star_mul
 
 end Star