The former London Irish, Harlequins and Stade Francais star had been plying his trade for San Isidro in his home country.
But the Exiles have brought the 27-year-old back to British rugby at the Kassam Stadium.
“The signing of Gonzalo Tiesi is a very significant one by the club,” said Welsh head coach Lyn Jones.
“He’s also a player of true international quality. He proved himself over the autumn period with Argentina.
“We look forward to Gonzalo adding a great deal to the playing and training environment at London Welsh.”
The 32-cap Puma has faced Wales on four occasions, scoring two tries. He also impressed in Argentina’s recent victory over Wales at the Millennium Stadium.
