The powerful scrummager, who has 33 international caps, will join the Club after the World Cup in Japan, if selected, later this year.
The 26-year-old, who currently plays for Jaguares and was part of the Argentina squad that reached the World Cup semi-finals in 2015, said he was setting his sights on helping Harlequins win trophies in their new era under Head of Rugby Paul Gustard.
He said: “It is brilliant to be joining a club with the drive and ambition that Paul Gustard and his team have at Harlequins. I know I am joining a club with a fantastic heritage and one that is moving in the right direction and with its sights set on winning silverware.”
Gustard added: “We are delighted to welcome Santi to Harlequins and know he will fit in brilliantly to the winning environment we have created at the Club.
“He is a powerful prop who has had an excellent career in Argentina to date and now we look forward to seeing him help us build on the progression we have made this season.
“He is a young man who has already played a lot of Super Rugby and international rugby and will add a significant high level of experience to our squad. He is mobile, aggressive and caught the eye of Bomb [Adam Jones] with his powerful scrummaging.
“He is joining us as he enters the prime of his career and I know everyone at Quins will welcome him and his girlfriend Maria to the Club as he looks to test himself in different competitions and develop his game further.”
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