Al-Sadr said he would cooperate with the new government on the condition that it provide a deadline for the end of the US-led occupation of Iraq.
"I support the new interim government...help me take this society to the path of security and peace," he said in a statement read by an aide before a Friday prayer congregation in Kufa.
"Starting now, I ask you that we open a new page for Iraq and for peace," his statement read.
Al-Sadr had earlier vehemently opposed the nomination of Shia secularist Iyad Allawi as Iraq's interim prime minister but appears to have bowed to pressure from other Shia leaders.
Meanwhile, AFP reported that Friday prayers in Najaf were halted when rival Shia factions allegedly threw stones at one another.
Witnesses said Al-Sadr supporters clashed with members of the Supreme Council of the Islamic Revolution in Iraq (SCIRI) as they entered the Imam Ali mosque.
In Baghdad, Sunni cleric Ahmad Hassan al-Taha used the Friday sermon to criticise the new transitional government as a by-product of US occupation.
He accused unnamed members of the transitional government of acting as proxy agents for the US State Department and Department of Defence.
"Iraq is heading to chaos," he said.