diff --git a/core/modules/filters/is/variable.js b/core/modules/filters/is/variable.js
new file mode 100644
index 000000000..fe7ea94bc
--- /dev/null
+++ b/core/modules/filters/is/variable.js
@@ -0,0 +1,36 @@
+/*\
+title: $:/core/modules/filters/is/variable.js
+type: application/javascript
+module-type: isfilteroperator
+
+Filter function for [is[variable]]
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.variable = function(source,prefix,options) {
+	var results = [];
+	if(prefix === "!") {
+		source(function(tiddler,title) {
+			if(!title in options.widget.variables) {
+				results.push(title);
+			}
+		});
+	} else {
+		source(function(tiddler,title) {
+			if(title in options.widget.variables) {
+				results.push(title);
+			}
+		});
+	}
+	return results;
+};
+
+})();
diff --git a/editions/tw5.com/tiddlers/filters/is.tid b/editions/tw5.com/tiddlers/filters/is.tid
index bd33d11a6..6e838e872 100644
--- a/editions/tw5.com/tiddlers/filters/is.tid
+++ b/editions/tw5.com/tiddlers/filters/is.tid
@@ -1,5 +1,5 @@
 created: 20140410103123179
-modified: 20161127142329969
+modified: 20190225130632157
 tags: [[Filter Operators]] [[Common Operators]] [[Negatable Operators]]
 title: is Operator
 type: text/vnd.tiddlywiki
@@ -22,6 +22,7 @@ The parameter <<.place C>> is one of the following fundamental categories:
 |^`system` |is a [[system tiddler|SystemTiddlers]], i.e. its title starts with `$:/` |
 |^`tag` |is in use as a tag |
 |^`tiddler` |exists as a non-shadow tiddler |
+|^`variable` |<<.from-version "5.1.20">> exists as a variable (whether or not it has a non-blank value) |
 
 If <<.place C>> is anything else an error message is returned. <<.from-version "5.1.14">> if <<.place C>> is blank, the output is passed through unchanged (in earlier versions an error message was returned).