The move, which is subject to a medical, will bring the centre to Scotstoun following two seasons with Munster, during which he started in the club's RaboDirect PRO12 play-off match with Warriors in May.
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The 33-year-old - who has agreed the move before joining up with the Ireland squad next week for a training camp in France - has also spent time previously with club sides Leinster, Connacht, Italian side Calvisano and Northampton Saints. The latter move comprised the most prosperous spell of his career, with Downey making 148 appearances and scoring 13 tries for the club, and winning the European Challenge Cup in 2009. He also triumphed in the Churchill Cup with Ireland in the same year.
He acknowledged yesterday that a move to Warriors was a "new challenge" but does not expect to be too far from success at the Scottish side. "The squad gets stronger each year and reaching the RaboDirect PRO12 final for the first time last season shows the progress the club is making. I know I'll have to work hard to get into the team," said Downey.
"I'm looking forward to playing at Scotstoun regularly in front of very passionate supporters. The atmosphere at the RaboDirect PRO12 play-off was incredible. I can't wait to start pre-season training."
The Irishman was also welcomed by Gregor Townsend, the Glasgow head coach. "James is a quality player who will be an excellent addition to our squad," he said. "He is a tough defender and brings real physicality when carrying the ball."