JkDefrag is a free open-source disk defragmenting utility computer program for Windows.

It was developed by Jeroen Kessels (initials "JK", hence the name) beginning in 2004[1] and was released under the GNU General Public License.

MyDefrag, which was closed source freeware, was discontinued, with the last version being v4.3.1, file date 21 May 2010; for several years the MyDefrag website has been a redirect to the still-existing JkDefrag site, but links to downloadable JKdefrag files are dead.

However such use would be completely unsupported and might bear the risk of data loss or a corrupt OS or other severe problems.

There are several graphical user interface utilities designed to work with JkDefrag: The only other Windows-based defrag program with a GNU GPL is UltraDefrag before version 8.