diff --git a/versionwarning.js b/versionwarning.js
new file mode 100644
index 0000000000000..651e20505faf6
--- /dev/null
+++ b/versionwarning.js
@@ -0,0 +1,34 @@
+(function() {
+    var latestStable = '0.21';
+    var goodPaths = ['stable', 'dev', latestStable];
+    var showWarning = (msg) => {
+        $('.body[role=main]').prepend(
+            '<p style="' +
+            [
+                'padding: 1em',
+                'font-size: 1.5em',
+                'border: 1px solid red',
+                'background: pink',
+                'width: 100%',
+            ].join('; ') +
+            '">' + msg + '</p>')
+    };
+    if (location.hostname == 'scikit-learn.org') {
+        var versionPath = location.pathname.split('/')[1];
+        if (!goodPaths.includes(versionPath)) {
+            showWarning('This is documentation for an old release of ' +
+                        'Scikit-learn (version ' + versionPath + '). Try the ' +
+                        '<a href="https://scikit-learn.org">latest stable ' +
+                        'release</a> (version ' + latestStable + ') or ' +
+                        '<a href="https://scikit-learn.org/dev">development</a> ' +
+                        '(unstable) versions.')
+        } else if (versionPath == 'dev') {
+            showWarning('This is documentation for the unstable ' +
+                        'development version of Scikit-learn. (To use it, ' +
+                        '<a href="https://scikit-learn.org/stable/developers/advanced_installation.html#installing-nightly-builds">install the nightly build</a>.) ' + 
+                        'The latest stable ' +
+                        'release is <a href="https://scikit-learn.org">version ' +
+                        latestStable + '</a>.')
+        }
+    }
+})()