Foster's expected departure from Saints was confirmed days after reports emerged of a training-ground disagreement with manager Tommy Wright.
And the former Aberdeen and Rangers player quickly signed for County until the end of the season, returning to the club with whom he won the League Cup in 2016 during his only previous campaign in Dingwall.
The 34-year-old told County's Twitter account: "When the opportunity arose to come back up, it was one that I snatched.
"I had spoken to my agent previously and and said to him that Ross County was top of my list, and happily the manager here phoned up my agent and said 'we would like him to come back'. So it was great for me.
"As I'm coming up here and looking through all the pictures, seeing the ones of us celebrating, it was a great part of my career.
"That was part of the draw to come back, it was a club where I had a lot of success and met a lot of great people.
"It's a very welcoming club, very family oriented, and my family and my wife enjoyed coming here to watch games. So that was also a factor."
Saints earlier confirmed Foster had "left the club by mutual consent".
The statement added: "Richard joined Saints in August 2016 and made 112 appearances. The club thanks Richard for his service during his time as a player and wishes him well in his future career."