diff --git a/toolchain/check/handle_class.cpp b/toolchain/check/handle_class.cpp
index b7c6018eab096..4d62a334185b5 100644
--- a/toolchain/check/handle_class.cpp
+++ b/toolchain/check/handle_class.cpp
@@ -526,6 +526,21 @@ auto HandleParseNode(Context& context, Parse::BaseDeclId node_id) -> bool {
     return true;
   }
 
+  // TODO: base must appear before virtual functions too
+  for (auto inst_id : context.inst_block_stack().PeekCurrentBlockContents()) {
+    if (auto function_decl =
+            context.insts().TryGetAs<SemIR::FunctionDecl>(inst_id)) {
+      auto& function = context.functions().Get(function_decl->function_id);
+      if (function.virtual_modifier == SemIR::Function::VirtualModifier::Impl) {
+        CARBON_DIAGNOSTIC(
+            BaseDeclAfterImplDecl, Error,
+            "`base` declaration must appear before impl function declarations");
+        context.emitter().Emit(node_id, BaseDeclAfterImplDecl);
+        return true;
+      }
+    }
+  }
+
   auto base_info = CheckBaseType(context, base_type_node_id, base_type_expr_id);
 
   // TODO: Should we diagnose if there are already any fields?
diff --git a/toolchain/check/testdata/class/virtual_modifiers.carbon b/toolchain/check/testdata/class/virtual_modifiers.carbon
index 760b247e12293..8474eb28f2be2 100644
--- a/toolchain/check/testdata/class/virtual_modifiers.carbon
+++ b/toolchain/check/testdata/class/virtual_modifiers.carbon
@@ -79,9 +79,10 @@ class C {
 package FailModifiers;
 
 class C {
-  // CHECK:STDERR: fail_modifiers.carbon:[[@LINE+3]]:3: error: impl without base class [ImplWithoutBase]
+  // CHECK:STDERR: fail_modifiers.carbon:[[@LINE+4]]:3: error: impl without base class [ImplWithoutBase]
   // CHECK:STDERR:   impl fn F();
   // CHECK:STDERR:   ^~~~~~~~~~~~
+  // CHECK:STDERR:
   impl fn F();
 }
 
@@ -118,6 +119,25 @@ class Derived {
   impl fn F();
 }
 
+// --- fail_impl_before_base.carbon
+
+package ImplBeforeBase;
+
+base class Base {
+}
+
+class Derived {
+  // CHECK:STDERR: fail_impl_before_base.carbon:[[@LINE+4]]:3: error: impl without base class [ImplWithoutBase]
+  // CHECK:STDERR:   impl fn F();
+  // CHECK:STDERR:   ^~~~~~~~~~~~
+  // CHECK:STDERR:
+  impl fn F();
+  // CHECK:STDERR: fail_impl_before_base.carbon:[[@LINE+3]]:3: error: `base` declaration must appear before impl function declarations [BaseDeclAfterImplDecl]
+  // CHECK:STDERR:   extend base: Base;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~
+  extend base: Base;
+}
+
 // CHECK:STDOUT: --- modifiers.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -695,3 +715,56 @@ class Derived {
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl fn @F();
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_impl_before_base.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %Base: type = class_type @Base [template]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [template]
+// CHECK:STDOUT:   %complete_type.1: <witness> = complete_type_witness %empty_struct_type [template]
+// CHECK:STDOUT:   %Derived: type = class_type @Derived [template]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [template]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [template]
+// CHECK:STDOUT:   %ptr: type = ptr_type <vtable> [template]
+// CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr} [template]
+// CHECK:STDOUT:   %complete_type.2: <witness> = complete_type_witness %struct_type.vptr [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Base = %Base.decl
+// CHECK:STDOUT:     .Derived = %Derived.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Base.decl: type = class_decl @Base [template = constants.%Base] {} {}
+// CHECK:STDOUT:   %Derived.decl: type = class_decl @Derived [template = constants.%Derived] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Base {
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [template = constants.%complete_type.1]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Base
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Derived {
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [template = constants.%F] {} {}
+// CHECK:STDOUT:   %Base.ref: type = name_ref Base, file.%Base.decl [template = constants.%Base]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %struct_type.vptr [template = constants.%complete_type.2]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Derived
+// CHECK:STDOUT:   .F = %F.decl
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl fn @F();
+// CHECK:STDOUT:
diff --git a/toolchain/diagnostics/diagnostic_kind.def b/toolchain/diagnostics/diagnostic_kind.def
index 97cd57425936b..1263124316480 100644
--- a/toolchain/diagnostics/diagnostic_kind.def
+++ b/toolchain/diagnostics/diagnostic_kind.def
@@ -221,6 +221,7 @@ CARBON_DIAGNOSTIC_KIND(BaseDeclRepeated)
 CARBON_DIAGNOSTIC_KIND(BaseIsFinal)
 CARBON_DIAGNOSTIC_KIND(BaseMissingExtend)
 CARBON_DIAGNOSTIC_KIND(BaseDeclAfterFieldDecl)
+CARBON_DIAGNOSTIC_KIND(BaseDeclAfterImplDecl)
 CARBON_DIAGNOSTIC_KIND(ClassAbstractHere)
 CARBON_DIAGNOSTIC_KIND(ClassForwardDeclaredHere)
 CARBON_DIAGNOSTIC_KIND(ClassSpecificDeclOutsideClass)