Phillips is still recovering from a serious knee injury but is expected to be back in action at the end of September, leaving him plenty of time to press his claims for a place in Ian McGeechan's squad for the 2009 tour.
Marshall, who joined the Ospreys from Leeds in April 2006 and went on to play all three Tests for the All Blacks against the Lions in 2005, will join former Leicester and Lions centre Ollie Smith at Montpellier.
"Justin is exactly what we were looking for," said Montpellier coach Didier Nourault.
"He's a player of a very high standard with a lot of experience. He should bring plenty to the team and allow our young number nines, Julien and Adrien Tomas, to make progress."