diff --git a/prism/diagnostic.c b/prism/diagnostic.c
index 7cffce7c9f..925ccce4f0 100644
--- a/prism/diagnostic.c
+++ b/prism/diagnostic.c
@@ -255,6 +255,7 @@ static const char* const diagnostic_messages[PM_DIAGNOSTIC_ID_LEN] = {
     [PM_ERR_UNTIL_TERM]                         = "expected an `end` to close the `until` statement",
     [PM_ERR_VOID_EXPRESSION]                    = "unexpected void value expression",
     [PM_ERR_WHILE_TERM]                         = "expected an `end` to close the `while` statement",
+    [PM_ERR_WRITE_TARGET_IN_METHOD]             = "dynamic constant assignment",
     [PM_ERR_WRITE_TARGET_READONLY]              = "immutable variable as a write target",
     [PM_ERR_WRITE_TARGET_UNEXPECTED]            = "unexpected write target",
     [PM_ERR_XSTRING_TERM]                       = "expected a closing delimiter for the `%x` or backtick string",
diff --git a/prism/diagnostic.h b/prism/diagnostic.h
index 079d409147..a1e0b6d0d8 100644
--- a/prism/diagnostic.h
+++ b/prism/diagnostic.h
@@ -247,6 +247,7 @@ typedef enum {
     PM_ERR_UNTIL_TERM,
     PM_ERR_VOID_EXPRESSION,
     PM_ERR_WHILE_TERM,
+    PM_ERR_WRITE_TARGET_IN_METHOD,
     PM_ERR_WRITE_TARGET_READONLY,
     PM_ERR_WRITE_TARGET_UNEXPECTED,
     PM_ERR_XSTRING_TERM,
diff --git a/prism/prism.c b/prism/prism.c
index 15195ea5cd..6f2f37113b 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -10685,6 +10685,9 @@ parse_write(pm_parser_t *parser, pm_node_t *target, pm_token_t *operator, pm_nod
             return (pm_node_t *) pm_constant_path_write_node_create(parser, (pm_constant_path_node_t *) target, operator, value);
         case PM_CONSTANT_READ_NODE: {
             pm_constant_write_node_t *node = pm_constant_write_node_create(parser, (pm_constant_read_node_t *) target, operator, value);
+            if (context_def_p(parser)) {
+                pm_parser_err_node(parser, (pm_node_t *) node, PM_ERR_WRITE_TARGET_IN_METHOD);
+            }
             pm_node_destroy(parser, target);
             return (pm_node_t *) node;
         }
diff --git a/test/prism/errors_test.rb b/test/prism/errors_test.rb
index 906dbac594..9b6cf55fd5 100644
--- a/test/prism/errors_test.rb
+++ b/test/prism/errors_test.rb
@@ -1971,6 +1971,13 @@ module Prism
       end
     end
 
+    def test_constant_assignment_in_method
+      source = 'def foo();A=1;end'
+      assert_errors expression(source), source, [
+        ['dynamic constant assignment', 10..13]
+      ]
+    end
+
     private
 
     def assert_errors(expected, source, errors, compare_ripper: RUBY_ENGINE == "ruby")