Senators passed the measure by 75 votes to one on Wednesday, after it had already cleared the Chamber of Deputies, Brazil's lower house, in May. The change is the biggest overhaul of workers' rights since Brazil's return to democracy in 1988 and is estimated to benefit more than 30 million workers.
The vote lands in the middle of a tight election campaign. Flávio Bolsonaro, the right-wing son of former President Jair Bolsonaro, won the first round of the presidential election with 47% of the vote, ahead of Lula on 45.2%, setting up a decisive second round on 25 October. Lula's supporters hope the reform will close the gap.
The measure's popularity appears to have reshaped the politics around it. One nationwide opinion poll found that almost 70% of Brazilians backed the change, and trade unions held rallies in its favour in recent weeks. Flávio Bolsonaro, who currently serves as a senator, had previously opposed the amendment but announced he would vote for it, saying: "Most Brazilian workers want to see the end of the six-day workweek." He was among the 75 senators voting yes.