The 27-year-old has previously had Aviva Premiership spells with London Irish and Harlequins, in addition to playing for French club Stade Francais.
He made his international debut in 2004, going on to feature in the last two World Cups, and has so far won 32 caps. He was part of the Argentina side that beat Wales in Cardiff last November.
Tiesi will move to London Welsh from the Buenos Aires-based San Isidro club.
"The signing of Gonzalo Tiesi is a very significant one by the club," London Welsh head coach Lyn Jones said.
"He is a proven and very experienced Premiership player who has had positive spells at London Irish and Harlequins in the past.
"He's also a player of true international quality and he proved himself again over the autumn period with Argentina.
"We look forward to Gonzalo adding a great deal to the playing and training environment at London Welsh."
- Sports & Recreation
- Gonzalo Tiesi
- London Welsh