Al-Sadr's group announced a boycott at the end of last year to press for a withdrawal of US troops from Iraq and to protest against a meeting between al-Maliki and George Bush, the US president.
The al-Sadr movement held a joint news conference on Sunday with members of the Shia Alliance, to which the group belongs, to announce their return to parliament.
"One cannot believe that the Shia-Sunni civil war was not pre-planned by the West"
Mohammad Karim, Melbourne, Australia
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Bahaa al-Araji, a senior member of the al-Sadr group, said: "Since there has been a response to our demands, we declare that we will attend parliament today."
Mahmoud al-Mashadani, a parliamentary speaker, said that all the parliamentary parties would form a committee to discuss the reasons for the boycott in an attempt to resolve the issues.
"This is a new beginning," he said. "We want to say to the world that an Iraqi solution for Iraqi problems is the key, and others must support these solutions."