La Rochelle stunned as Brive relegated from Top 14
Defeated ended La Rochelle's run of 10 straight wins but it had little effect domestically as they remain in second place and qualified for the semi-finals.
With a place in the play-offs already secured, coach Ronan O'Gara took the opportunity to rest a host of key players ahead of their Champions Cup final in Dublin.
For reigning French champions Montpellier, the win ended a run of three successive losses.
Hooker Vincent Giudicelli (two), South African centre Jean Serfontein and fly-half Louis Carbonel all crossed for Montpellier in a devastating first-half display.
Remi Bourdeau, Sacha Idoumi and Hoani Bosmorin ran in tries in the second period as La Rochelle cut the deficit to 35-31.
However, Montpellier held their nerve to add two more scores from back-rowers Alexandre Becognee and Clement Doumenc.
Brive, who were European champions in 1997, slipped into the second division after a 16-13 home defeat by Castres.
"We fought like dogs even when we were 13 against 15," said Castres coach Jeremy Davidson, reflecting on two yellow cards handed out to his side.
Despite the win, former Ireland international Davidson felt for Brive despite being sacked as their coach last October after four years in charge.
"I have no grudge against Brive. Professional sport is like this. I brought this team up to the Top 14, seeing them go down to Pro D2 makes me sad," he said.
An unexpected 27-26 victory over Montpellier last weekend had given Brive hope of escaping the drop but Saturday's insipid performance saw them leave the top flight for the first time in four years.