Tamarin is a discontinued free software virtual machine with just-in-time compilation (JIT) support intended to implement the 4th edition of the ECMAScript (ES4) language standard.

[2] The source code, including the just-in-time compiler and conservative garbage collector, was donated to the Mozilla Foundation on November 7, 2006.

[3] The contributed code is tri-licensed under the GPL, LGPL, and MPL licenses to be developed in Mozilla's Mercurial repository.

The only part of Tamarin used in modern Mozilla applications (i.e. Firefox 3.5+) via SpiderMonkey is NanoJIT, a module that is used to generate native code when performing just-in-time compilation.

Two projects related to Tamarin were announced on July 25, 2007, in Brendan Eich's keynote at The Ajax Experience West: IronMonkey and ScreamingMonkey.