CURITIBA, Brazil—A convicted money launderer at the heart of an investigation into an alleged corruption scheme at Brazil’s state-run oil company Petroleo Brasileiro SA alleged President Dilma Rousseff and former President Luiz Inácio Lula da Silva knew about the alleged scheme.

Alberto Youssef, a currency dealer who was convicted of money laundering and sentenced last month to three years in prison, made the allegations to a congressional commission investigating the alleged corruption at Petrobras. Mr. Youssef had previously...