Ruaridh will join up on a one-year deal after spells with Wasps and Harlequins.
Ruaridh, who can play at fly-half or full-back, made his debut for the club in 2008.
He went on to become an integral member of the Warriors squad, helping the Scotstoun side reach their first ever Guinness PRO12 Final, as well as being involved in some of Glasgow's most memorable European victories.
He played 19 times for Wasps and scored a memorable try against Toulon in a 32-6 win over the French Top 14 side in the European Rugby Champions Cup.
Ruaridh has also represented his country 31 times to date and was part of the Scotland squad for the 2011 Rugby World Cup. His last appearance for Scotland came earlier this summer, scoring a try in the 27-22 defeat to Fiji.
Speaking to glasgowwarriors.org, Ruaridh said: "I'm really excited about coming back to Glasgow. I spent a long time there earlier in my career and it was great.
"Playing in England has been a good experience and one that has made me a better and more rounded player.
"I enjoyed working under Jason O'Halloran when he was with Scotland, so I'm looking forward to striking up that relationship again. I'm really excited to see how Dave Rennie operates.
"I still see myself as a 10, but I'm happy to play either there or at full-back.
"I've obviously got a few good friends still at the club, but a lot of young boys have come through over the last few years, so it's a bit different on the playing side.
"It's a really exciting time to be re-joining, with the new Guinness PRO14 and all it holds, so I'm looking forward to getting back up to Glasgow."
Glasgow Warriors Assistant Coach, Jason O'Halloran added: "Ruaridh is an experienced player, who will add to the competition we have within our squad.
"He knows the club well from his previous time here and can play in a number of positions in the back-line.
"We're looking forward to welcoming Ruaridh back to the club later this week."