"Two explosive devices were thrown at the mosque, breaking its windows and damaging the facade," Mufti Edhem Camdzic said after the attack in the northern Serb-run town of Gradiska.
Bosnian Serb police confirmed the attack but refused to elaborate, pending further investigations. The attack came exactly 11 years after the mosque was flattened by Bosnian Serb forces in the republic's 1992-1995 war.
Three days earlier arsonists set fire to an Orthodox church in the Serb village of Cipuljic, in the mainly Muslim region around Bugojno, causing minor damage.
Bosnian police said last week they were on high alert amid violent clashes between Serbs and predominantly Muslim Albanians in the southern Serbian province of Kosovo.
The clashes erupted on 17 March but subsided on the weekend, leaving 28 people dead and some 600 injured, most of them from the minority Serb community.
Bosnia's war ended more than eight years ago but tensions between ethnic Serbs, Muslims and Croats are never far from the surface.