Byrne has won the United Rugby Championship four times and the Champions Cup during a 10-year spell with Leinster, during which he has accumulated 1,156 points.
He has played 22 times for Ireland, winning two Triple Crowns, one Six Nations championship and a Six Nations Grand Slam.
Byrne’s switch comes just weeks after his younger brother Harry also moved to the Premiership, joining Bristol from Leinster on loan.
“Leinster is quite rightly recognised as a breeding ground for lots of quality players, and Ross is one of the best players they’ve produced over the last decade,” said Gloucester director of rugby George Skivington.
“It’s rare to come across someone with so much domestic and international pedigree, and when they do become available, inevitably there’s a lot of interest in them, so it’s a real coup for us.
“Ross’s signing is a real statement of ambition, and we can’t wait for him to get started.”
