Tonetti, who turned 23 last month, played for the Ireland Under-20s in their Grand Slam-winning 2007 campaign, and also featured for Ireland at last year's IRB Rugby World Cup Sevens tournament.
A regular starter for Blackrock College in the AIB League, he made his debut for Leinster against the Newport Gwent Dragons in May 2009, and made two more Magners League appearances against Connacht and Glasgow Warriors this past April.
Sale head coach Mike Brewer said: "I worked with Kyle during my time at Leinster and I believe him to be a great prospect with plenty of potential.
"He has started training with us at Carrington and I am sure he will be a great addition to our squad over the coming months."
Sale CEO Mick Hogan added: "Kyle is a welcome addition to the squad in our rebuilding process at the club.
" The signings we have announced so far have all been well received and supporters are looking forward to the new season which is only just over ten weeks away."