The jihadists have put up signs in Mosul's streets announcing the new "mujahadeen" mosque, Iraqi news website Ankawa reported, citing local witnesses.
IS militants chiselled off a large stone cross that adorned the Syrian Orthdox Sant'Efrem church's facade and put up a massive banner across it bearing the group's black and white Salafite logo.
"There is no God but Allah," read a slogan scrawled on the banner with "Prophet Mohammed" written below. IS last November removed and demolished the cross on the cupola of Sant'Efrem and tried to sell all of its furnishings and decorations, according to Ankawa.
The group overran Sant'Efrem -- the largest church in Mosul -- last July.
IS fighters have destroyed most of the art and religious monuments in Mosul, whose Christian and other minority populations have fled since the Sunni extremist group captured the city last June.