The Chile international, who can play anywhere on the right flank, joins Harry Redknapp's newly promoted side for a season, becoming their fourth signing of the summer
The versatile Chilean, who can play anywhere on the right flank, made 29 league appearances for the Old Lady after joining from Udinese, lifting the Serie A title twice.
Isla becomes Harry Redknapp's fourth signing of the summer, following the arrivals of Rio Ferdinand, Steven Caulker and Jordon Mutch.
"He's a top player and we're delighted to have him here," the veteran manager told the club's offial website.
"He showed his quality in the World Cup for Chile and it's a great loan signing for us. He's a versatile player, he's full of energy. I'm confident he'll be a really big asset for us this season."