Foster was one of the 11 senior players who were allowed to leave Ibrox after their contracts expired at the end of last season.
Rangers chairman Dave King later criticised the group of departing stars, labelling them failures after they were beaten in the play-offs by Motherwell, consequently missing out on promotion.
Foster had returned to Glasgow for a second spell with the fallen Scottish giants in 2013 after spending most of the 2010-11 season on loan there from Aberdeen.
In total, the former Scotland U21 international played 82 games for Rangers over two different periods but failed to score.
Foster, who has also played for Bristol City, comes into Jim McIntyre’s squad at Victoria Park with former Preston and Port Vale defender Chris Robertson to bring the number of summer signings there to nine.
Jonathan Franks, Scott Fox, Ian McShane, Stewart Murdoch, Brian Graham and Dean Holden have all agreed terms with the Highland side.
Michael Gardyne, who was on loan to the Dingwall outfit last season from Dundee United, has also come back on a permanent basis.
Bradford City defender Andrew Davies is another player heading to County ahead of the Premiership kick-off. The 30-year-old will start the season as Staggies skipper after also captaining City in League 1.
Bantams boss Phil Parkinson said: “We thank him for what he has done for this club – he’s been a tremendous player during his time here.
“But that’s the way football is, things move on.”