The movement's 32 MPs walked out of parliament after a second bomb attack on a revered Shia shrine in central Iraq, which al-Sadr blamed on the failure of Iraqi security forces protecting it.
Tuesday's move was welcomed by the Iraqi parliament.
Acting house speaker, Khalid al-Atiyya, said: "We welcome back the members of the Sadr bloc into the council of parliament to practise their legislative role, especially at this stage."
Wave of fighting
Al-Sadr withdrew six ministers from government in April over the refusal of Nuri al-Maliki, the Iraqi prime minister, to support a timetable for US withdrawal.
Al-Sadr's bloc also withdrew from parliament for two months in November after al-Maliki met George Bush, the US president, in Amman.
The political bloc of al-Sadr demanded that the al-Askari shrine in the town of Samarra be rebuilt, after its golden dome was blown up in February 2006.
The attack ignited a wave of sectarian fighting that has killed tens of thousands of people.
The group stepped up its protest after the shrine was bombed again in June.