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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.