The 27-year-old has moved to the club on the back of stints at Harlequins, London Irish and Buenos Aires side San Isidro Club.
London Welsh head coach Lyn Jones told the club's official website: "The signing of Gonzalo Tiesi is a very significant one by the club. He's 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."
Tiesi has racked up 32 caps for Argentina.