iSync is a defunct application developed by Apple Inc., which syncs iCal and Address Book data to SyncML-enabled mobile phones, via Bluetooth or by using a USB connection.

iSync uses port 3004, which could also be blocked if the Mac OS X firewall was enabled.

[4][5] Before the release of Mac OS X 10.4, iSync also synchronized a user's Safari bookmarks with the now-defunct .Mac subscription service provided by Apple.

iSync, however, retained responsibility for the setup, configuration and synchronising of supported mobile handsets.

[12] In 2005, iSync supported iPods, personal digital assistants (PDAs), mobile phones, and other devices;[13] iSync supported phones from Motorola, Nokia, Panasonic, Sony Ericsson, Siemens, and Sendo.