diff --git a/toolchain/check/impl.cpp b/toolchain/check/impl.cpp
index bf3e171f68a9..d2642f05b237 100644
--- a/toolchain/check/impl.cpp
+++ b/toolchain/check/impl.cpp
@@ -147,7 +147,7 @@ static auto ScopesMatch(Context& context, const SemIR::Impl& new_impl,
 
   // The redecl is is an invalid scope.
   CARBON_DIAGNOSTIC(ImplDeclInInvalidScope, Error,
-                    "impl redeclation not in a declarative scope; "
+                    "impl redeclaration not in a declarative scope; "
                     "redeclaration is allowed only in a class or namespace");
   context.emitter().Emit(new_impl.latest_decl_id(), ImplDeclInInvalidScope);
   return ImplRedeclType::DiagnosedInvalidRedecl;
diff --git a/toolchain/check/testdata/impl/redeclaration.carbon b/toolchain/check/testdata/impl/redeclaration.carbon
index c8b903fd78e5..217725a74eeb 100644
--- a/toolchain/check/testdata/impl/redeclaration.carbon
+++ b/toolchain/check/testdata/impl/redeclaration.carbon
@@ -246,7 +246,7 @@ interface Z {}
 fn F() {
   class C {}
   impl C as Z;
-  // CHECK:STDERR: fail_redecl_in_fn_scope.carbon:[[@LINE+8]]:3: error: impl redeclation not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
+  // CHECK:STDERR: fail_redecl_in_fn_scope.carbon:[[@LINE+8]]:3: error: impl redeclaration not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
   // CHECK:STDERR:   impl C as Z {}
   // CHECK:STDERR:   ^~~~~~~~~~~~~
   // CHECK:STDERR:
@@ -268,7 +268,7 @@ fn F() {
 }
 
 fn G() {
-  // CHECK:STDERR: fail_redecl_in_different_fn_scope.carbon:[[@LINE+8]]:3: error: impl redeclation not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
+  // CHECK:STDERR: fail_redecl_in_different_fn_scope.carbon:[[@LINE+8]]:3: error: impl redeclaration not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
   // CHECK:STDERR:   impl C as Z {}
   // CHECK:STDERR:   ^~~~~~~~~~~~~
   // CHECK:STDERR:
@@ -288,7 +288,7 @@ fn F() {
   if (true) {
     class C {}
     impl C as Z;
-    // CHECK:STDERR: fail_redecl_in_block_scope.carbon:[[@LINE+8]]:5: error: impl redeclation not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
+    // CHECK:STDERR: fail_redecl_in_block_scope.carbon:[[@LINE+8]]:5: error: impl redeclaration not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
     // CHECK:STDERR:     impl C as Z {}
     // CHECK:STDERR:     ^~~~~~~~~~~~~
     // CHECK:STDERR:
@@ -311,7 +311,7 @@ fn F() {
     impl C as Z;
   }
   if (true) {
-    // CHECK:STDERR: fail_redecl_in_different_block_scope.carbon:[[@LINE+8]]:5: error: impl redeclation not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
+    // CHECK:STDERR: fail_redecl_in_different_block_scope.carbon:[[@LINE+8]]:5: error: impl redeclaration not in a declarative scope; redeclaration is allowed only in a class or namespace [ImplDeclInInvalidScope]
     // CHECK:STDERR:     impl C as Z {}
     // CHECK:STDERR:     ^~~~~~~~~~~~~
     // CHECK:STDERR: