diff --git a/HISTORY.md b/HISTORY.md
index 5ce7214e..6a49a059 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -56,6 +56,7 @@
 * threads: made block attribute "type" case-insensitive for textual mnemonics
 * threads: allow variadic slot type declaration using ellipses after type numbers
 * threads: allow case-insensitive text for custom block scope specification
+* threads: update an existing global definition matching DEFINE's label
 
 ### 2022-06-23
 * objects, threads: turned DEFINE into a command block with an upvar, experimental
diff --git a/src/threads.js b/src/threads.js
index 55c4759a..fe2ee023 100644
--- a/src/threads.js
+++ b/src/threads.js
@@ -5990,7 +5990,7 @@ Process.prototype.doDefineBlock = function (upvar, label, context) {
         vars = this.context.outerContext.variables,
         type = this.reportTypeOf(context),
         count = 1,
-        spec, def;
+        matches, spec, def;
 
     this.assertType(label, 'text');
     if (label === '') {return ''; }
@@ -6002,6 +6002,29 @@ Process.prototype.doDefineBlock = function (upvar, label, context) {
         this.compileBlockReferences(context, upvar);
     }
 
+    // identify global custom block matching the specified label
+    matches = ide.stage.globalBlocks.filter(def =>
+        def.abstractBlockSpec() === label
+    );
+    if (matches.length > 1) {
+        throw new Error(
+            'several block definitions already match this label'
+        );
+    } else if (matches.length === 1) {
+        // update the existing global definition with the context body
+        def = matches[0];
+        this.doSetBlockAttribute(
+            'definition',
+            def.blockInstance().reify(),
+            context
+        );
+
+        // create the reference to the new block
+        vars.addVar(upvar);
+        vars.setVar(upvar, def.blockInstance().reify());
+        return;
+    }
+
     // make a new custom block definition
     def = new CustomBlockDefinition('BYOB'); // haha!
     def.type = type;