The 24-year-old made 22 appearances for Glasgow, but has not featured this year due to a series of injuries.
The New Zealand-born Samoa U20 international leaves seven months before his contract expires having spent two years at Scotstoun.
"It's a decision that's in the best interest of both the player and the club," head coach Dave Rennie said.
"Brian's had a number of injuries and we've not been able to play him as often as we would have liked, but he's a good man, is very popular, and will be missed."