Mackintosh has signed for the French club as injury cover and started for the side in its European club tie against Cardiff Blues in the weekend.
The big loosehead prop, who has played more than 100 games for Southland, was left unsigned by any Super rugby team for next season, following a two-season spell at the Chiefs.
He had played more than 60 games for the Highlanders but left at the end of the 2013 season and joined the Hamilton-based franchise.
The 30-year-old played a limited role with the Chiefs last season and looked to be limited to playing for Southland next season.
He played for the Barbarians last month against Argentina in the United Kingdom and has linked up with Montpellier as injury cover as the Jake White-coached team goes through something of an injury crisis.
Mackintosh has signed a short-term contract but it may be extended until the end of the season depending on the fitness of other players.
Mackintosh scored a try for Montpellier over the weekend against Cardiff but the side ended up losing 37-27 in the European Challenge Cup.
Springbok hardman Bismarck du Plessis plays for Montpellier.
Former Otago and Highlanders lock Tom Donnelly also plays for the side while Chris King, who played 100 games for the Highlanders, had been playing at Montpellier but has moved to Pau this season, the same club as All Blacks Colin Slade and Conrad Smith.
● New Zealand Rugby announced hardware business Mitre 10 would be the new naming sponsor for domestic competitions from next season.
The men's national provincial competition will now be known as the Mitre 10 Cup and the company will also sponsor Heartland rugby, the women's competition and the national under-19 tournament.
The deal with Mitre 10 runs through until 2020.
