diff --git a/doc/rules.md b/doc/rules.md
index f8ac9b1..dea5987 100644
--- a/doc/rules.md
+++ b/doc/rules.md
@@ -2,7 +2,7 @@
 
 # List of Rules
 
-This document describes all (57)
+This document describes all (58)
 available rules, what they check for, examples of
 what they warn for, and how to fix their warnings.
 
@@ -108,6 +108,7 @@ For example, as follows:
 -   [no-literal-urls](#no-literal-urls)
 -   [no-missing-blank-lines](#no-missing-blank-lines)
 -   [no-multiple-toplevel-headings](#no-multiple-toplevel-headings)
+-   [no-reference-like-url](#no-reference-like-url)
 -   [no-shell-dollars](#no-shell-dollars)
 -   [no-shortcut-reference-image](#no-shortcut-reference-image)
 -   [no-shortcut-reference-link](#no-shortcut-reference-link)
@@ -1906,6 +1907,32 @@ When this rule is `1`, the following file
 3:1-3:6: Don’t use multiple top level headings (3:1)
 ```
 
+## `no-reference-like-url`
+
+Warn when URLs are also defined identifiers.
+
+When this rule is turned on, the following file
+`valid.md` is ok:
+
+```markdown
+[Alpha](http://example.com).
+
+[bravo]: https://example.com
+```
+
+When this rule is turned on, the following file
+`invalid.md` is **not** ok:
+
+```markdown
+[Charlie](delta).
+
+[delta]: https://example.com
+```
+
+```text
+1:1-1:17: Did you mean to use `[delta]` instead of `(delta)`, a reference?
+```
+
 ## `no-shell-dollars`
 
 Warn when shell code is prefixed by dollar-characters.
diff --git a/packages/remark-lint/lib/rules.js b/packages/remark-lint/lib/rules.js
index dd4d986..217995b 100644
--- a/packages/remark-lint/lib/rules.js
+++ b/packages/remark-lint/lib/rules.js
@@ -43,6 +43,7 @@ module.exports = {
   'no-literal-urls': require('./rules/no-literal-urls.js'),
   'no-missing-blank-lines': require('./rules/no-missing-blank-lines.js'),
   'no-multiple-toplevel-headings': require('./rules/no-multiple-toplevel-headings.js'),
+  'no-reference-like-url': require('./rules/no-reference-like-url.js'),
   'no-shell-dollars': require('./rules/no-shell-dollars.js'),
   'no-shortcut-reference-image': require('./rules/no-shortcut-reference-image.js'),
   'no-shortcut-reference-link': require('./rules/no-shortcut-reference-link.js'),
diff --git a/packages/remark-lint/lib/rules/no-reference-like-url.js b/packages/remark-lint/lib/rules/no-reference-like-url.js
new file mode 100644
index 0000000..6dc3ed6
--- /dev/null
+++ b/packages/remark-lint/lib/rules/no-reference-like-url.js
@@ -0,0 +1,70 @@
+/**
+ * @author Titus Wormer
+ * @copyright 2016 Titus Wormer
+ * @license MIT
+ * @module no-reference-like-url
+ * @fileoverview
+ *   Warn when URLs are also defined identifiers.
+ *
+ * @example {"name": "valid.md"}
+ *
+ *   [Alpha](http://example.com).
+ *
+ *   [bravo]: https://example.com
+ *
+ * @example {"name": "invalid.md", "label": "input"}
+ *
+ *   [Charlie](delta).
+ *
+ *   [delta]: https://example.com
+ *
+ * @example {"name": "invalid.md", "label": "output"}
+ *
+ *   1:1-1:17: Did you mean to use `[delta]` instead of `(delta)`, a reference?
+ */
+
+'use strict';
+
+/* Dependencies. */
+var position = require('unist-util-position');
+var visit = require('unist-util-visit');
+
+/* Expose. */
+module.exports = noReferenceLikeURL;
+
+/**
+ * Warn when unused definitions are found.
+ *
+ * @param {Node} ast - Root node.
+ * @param {File} file - Virtual file.
+ */
+function noReferenceLikeURL(tree, file) {
+  var identifiers = [];
+
+  visit(tree, 'definition', find);
+
+  visit(tree, 'image', check);
+  visit(tree, 'link', check);
+
+  return;
+
+  /* Find identifiers. */
+  function find(node) {
+    if (!position.generated(node)) {
+      identifiers.push(node.identifier.toLowerCase());
+    }
+  }
+
+  /* Check `node`. */
+  function check(node) {
+    var url = node.url;
+
+    if (identifiers.indexOf(url.toLowerCase()) !== -1) {
+      file.message(
+        'Did you mean to use `[' + url + ']` instead of ' +
+        '`(' + url + ')`, a reference?',
+        node
+      );
+    }
+  }
+}