The country's lower congress must decide whether Rousseff should face an impeachment trial on charges of manipulating budgetary accounts.
If 342 of the lower congress' 513 lawmakers vote for the impeachment, then the proceedings will move to the senate. Here a separate vote could suspend Rousseff, paving the way for vice president Michel Temer to take the top job.
Hundreds of thousands of protesters, for and against, took to the streets of major cities across the country ahead of the vote.
Rousseff is accused of manipulating public finances to boost her prospects for re-election in 2014.
She's also been tainted by the corruption scandal involving billions of dollars in bribes at state-run oil company Petrobas. Last month, Rousseff's predecessor, Luiz Inacio Lula da Silva, was taken in for questioning regarding the inquiry.
Read more: Protests grow as Brazil's crisis deepens
This has all taken place against a backdrop of the South American country's worst economic crisis in decades.
Polls suggest more than 60 per cent of Brazil's 200m people support impeaching Rousseff.
Today's vote started at 5pm GMT and is expected to run into the evening.