Argentina scrum-half Martin Landajo will join Harlequins from Jaguares after the Rugby World Cup.
The 30-year-old playmaker, capped 84 times by the Pumas, is expected to make his debut for the Premiership club in November.
Landajo will give Paul Gustard another high-quality option at scrum-half option along with Danny Care at The Stoop.
"I cannot wait to get started with Harlequins," said Landajo. "I have always admired the club's history and place in the English game and, under Paul Gustard, I can see they have become even stronger and are now in a great place to win trophies."
Gustard said: "Martin will be a wonderful addition to the squad and add greater density to our leadership group.
"They call him The Door Opener - and for good reason. His pace and creativity has been able to unlock defences at the highest level and we look forward to welcoming him at The Stoop later this year.
"He is a very impressive man who is driven to push on, test himself in a different environment and leave his own personal legacy in the game.
"We are delighted he is committing his future with us and we look forward to welcoming Martin and his young family to Quins."