There has been a political crisis since President Pierre Nkurunziza announced he would run for a third term.
Mr Rwasa withdrew from this month’s presidential election and described Mr Nkurunziza’s victory as “a joke”.
He now says he will “play the game” to bring peace, AFP news agency reports.
Mr Rwasa supported the protests, that began in April, against Mr Nkurunziza’s third-term bid in which more than 70 people have died in clashes with the police.
There was also a failed coup attempt in May.
Mr Rwasa’s opposition coalition also called for a boycott of June’s parliamentary elections, but with its name on the ballot paper it still won 21 seats.
There is a faltering negotiation process, chaired by Uganda, that is aimed at solving the crisis.
Mr Rwasa took up his seat in parliament to some surprise on Monday saying that as the talks were still going on it was worth participating in the political process.
Fellow opposition leader Charles Nditije said Mr Rwasa’s move betrayed those who died during the protests.
He was elected as one of parliament’s deputy speakers with the backing of MPs from Mr Nkuruniziza’s CNDD-FDD party.
The BBC’s Prime Ndikumagenge in Burundi’s capital, Bujumbura, says that it is difficult to see what Mr Rwasa’s strategy is.
But the opposition leader may now have more influence on the politics of the country in his new post, he says.