Kennedy, 28, who had a short spell on loan at Glasgow seven years ago, will be available to play for County when he is not selected by the Warriors.
Capped by Scotland at Under-20 level and sevens, he received a senior international call-up ahead of the 2013 Six Nations but did not play.
His seven-year spell with Edinburgh came to an end last season after 66 appearances for the capital club. He has also played for London Irish, after coming through the ranks at Alloa and Stirling County.
Kennedy has been training with Glasgow this week and said: “It’s been really good to get back involved with the boys.
“When I was here on loan before I didn’t play that much, but I really enjoyed myself. The style of play that Glasgow favour was a massive factor in my decision to come back – obviously different coaches will have different focuses, but I loved watching the free-flowing attacking play and I want to be a part of that. It’s always been a part of the game I enjoy, trying to get to breakdowns as quickly as possible and keeping that tempo high.
“I came through the ranks at Stirling County, from Under-18s right up until I signed full-time with Scotland 7s, so it’ll be great to get back there too.
“With Super 6 on the way, I’m hoping we can do something special. I’m excited about it, especially the cross-border element of the competition. With boys being semi-pro now, the standard should be that bit higher, so it’s a great prospect.”
Glasgow Warriors assistant coach, Kenny Murray, said: “Sean is an exciting player who really suits our style of play. His speed around the pitch and the speed of his pass has the ability to really stretch defences. He’ll provide valuable depth in a position which we expect to be depleted in for large chunks of the season and will compete to be involved even when we are at full strength.”