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.

[11] Adobe continued to use Tamarin in its Flash Player, but it did not replace SpiderMonkey as the JavaScript engine of Mozilla applications.

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.