A spokesman for Sistani, Sayed Ahmed Safi, said any objections had been removed by last-minute changes to the text endorsed by parliamentary on Wednesday.
"The Iraqi faithful must go to the polling stations to vote in favor," he said.
For the minority Sunni Muslims, despite the changes made Wednesday, some leaders fear the current charter may lead the country to split into a Kurdish north and Shiite south, depriving Sunnis of access to the country's oil resources.
If voters in three of Iraq's 18 provinces muster a two-thirds majority against the constitution, it will fail. The Sunnis are dominant in four provinces and so hold a power of veto if they turn out in large numbers to vote against the constitution.