The 28-year-old, who is on replacement duty for Argentina against England at Twickenham this weekend, has agreed terms with Newcastle for the rest of this season.
Tiesi, capped 37 times, has previously played for the likes of London Irish, Harlequins and Stade Francais.
Newcastle rugby director Dean Richards said: "Gonzalo adds real quality to our squad, and I'm delighted to welcome him to Newcastle.
"The fact that he already has Premiership experience under his belt is a major plus for us, and his international experience is invaluable as we go into a series of big games in the coming months."
- Sports & Recreation