The iPhone app will be joined by one for BlackBerry handsets, which will be available from May, Skype said.
Skype has been a noticeable latecomer to the App Store, which features several other VoIP applications.
One of those, Fring, uses Skype's API (application programming interface). People can make free Skype-user-to-Skype-user calls, or use SkypeOut, a paid service where people can call landlines and mobiles via Skype.
The iPhone Skype application can only be used over Wi-Fi and not over cellular networks, a condition demanded by operators so as to not damage voice revenue. A spokeswoman would not reveal further details, saying the mobile application will have the "core capabilities" of the desktop application.
Skype does not yet have a native application for BlackBerry devices, although third-party developers again have utilised Skype's API. One of those services is IM+ for Skype, which enables chat, free calls to Skype users and SkypeOut. Skype did not provide the application's May launch date.
Skype's application for the Windows Mobile OS has been available for several years.
See also: Skype for iPhone coming next week?