The loose-head prop, who left Glasgow for the English Premiership last season, returns to Scotstoun where he played 109 times.
With 25 Scotland caps to his name, Ryan also toured Australia with the British and Irish Lions in 2013.
Robert has played for the Warriors in pre-season games against Canada 'A' and Northampton Saints and has also featured at five World Series tournaments with Scotland 7s.
The 24-year-old is capable of playing on the wing or at centre.
Ryan told glasgowwarriors.org: "The opportunity to come up here is one I took with both hands. Not many players are lucky enough to come back here after they leave, I consider myself very lucky to have already played over 100 times for the club and hopefully I can add to that.
"It was good to spend a little while playing in the Premiership in England, learning different techniques from different coaches, but it also made me appreciate what a great club Glasgow is and I'm grateful to be back.
"I stayed in contact with a lot of the guys, I'm good friends with a lot of the boys so I never really lost contact. My wife and I are delighted to be back in Glasgow."
Glasgow Warriors Assistant Coach Kenny Murray said: "Ryan has been a great servant to the club in the past. He's a very well liked player amongst the players and the coaches so he'll come and fit in really well.
"He's shown that he's really keen to get back involved so we're anticipating he'll take his opportunity and do really well.
"In Bobby we have another young local player coming through the system. He has been a late developer and his attitude is a great example to young players to keep working hard.
"He's played really well for Hawks this season and was one of the stand-out trainers at Glasgow during pre-season."