iSync is a software application published by Apple Inc. It runs only under Mac OS X and is used to synchronize data in iCal and the Address Book with .Mac and with devices including iPods, many SyncML-compatible mobile phones and Palm OS handheld organizers & smartphones. PocketPCs and Windows Mobile devices cannot be used with iSync, but are supported by third-party applications. Prior to the release of Mac OS X v10.4, iSync could also synchronize a user's Safari bookmarks with the .Mac subscription service provided by Apple.

Starting with Mac OS X v10.4, much of this functionality has been moved into the Sync Services framework, which developers can use to incorporate synchronization into their own applications. iSync, however, retains responsibility for synchronizing mobile handsets, Palm OS organizers and compatible smartphones. With the release of iTunes 4.8, the responsibility for synchronizing iPods under Mac OS X v10.4 has been delegated to iTunes. Synchronization with .Mac is now the domain of .Mac Sync, accessible through a System Preferences pane.