Palmeiras went down just four months after winning the Copa Brasil knockout competition and will play next year's South American Libertadores Cup while in Brazil's Serie B.
"Have you been to the wake? That's what's happening here," Palmeiras coach Gilson Kleina told reporters.
The club's relegation came after they drew 1-1 away to Flamengo, whose former Palmeiras striker Vagner Love equalised in the 89th minute of the match at Volta Redonda.
Portuguesa, in 16th two places above Palmeiras, also drew 2-2 at home to Gremio, leaving Palmeiras seven points adrift in the relegation zone with two matches remaining.
The bottom four teams in the championship are relegated. Figueirense and Goianiense already down. Sport, in 17th place one point behind Portuguesa, can still avoid the drop.
Palmeiras, who share the record of eight Brazilian league titles with Santos, were crowned South American champions when they won the Libertadores Cup in 1999.
They appeared to be on their way to only their fourth away win in the championship when Vinicius opened the scoring with a shot from outside the box in the 64th minute.
Flamengo then had substitute Paulo Sergio sent off in the 85th minute for a foul on Roman but Love put the final nail in their coffin four minutes later with a deflected shot.
"We tried everything but it was impossible. There's a feeling of impotence now. We needed to win, we apologise for everything, the fans were always behind us," Palmeiras' Argentine striker Hernan Barcos told reporters.
Before the match, played at the Volta Redonda due to World Cup refurbishments at the Maracana, police stopped a fight between fans of the rival teams.
Palmeiras fans have been involved in several violent incidents protesting against the team and the club.
In September with the team already in the relegation zone, Palmeiras parted company with former Brazil, Portugal and Chelsea coach Luiz Felipe Scolari - a mere two months after they had won the Copa Brasil.
Critics blamed Scolari, who steered Palmeiras to their 1999 Libertadores Cup victory in a previous spell in charge, for their relegation. They spent one season in the second division after their first relegation in 2002.
Fluminense, crowned champions last weekend with three matches to spare, were handed the trophy in a ceremony at the Engenhao in Rio, only to lose 2-0 to visiting Cruzeiro.
