Turkey's President Recep Tayyip Erdogan announced thedecision after a court annulled the site's museum status.
Built 1,500 years ago as an Orthodox Christian cathedral,Hagia Sophia was converted into a mosque after the Ottoman conquest in 1453.
In 1934 it became a museum and is now a Unesco WorldHeritage site.
Islamists in Turkey long called for it to be converted to amosque but secular opposition members opposed the move.
The proposal promptedcriticism from religious and political leaders worldwide.
Defending the decision, President Erdogan stressed that thecountry had exercised its sovereign right in converting it back to a mosque.