Meyer reels in Jaguares' talisman
Sanchez, 29, has been a star player over the last few years for Argentina and impressed at Toulon in the 2014-15 season after a three-year stint at Bordeaux-Begles.
He will now return to France after spending the last two seasons in his home country with Super Rugby team Jaguares.
The other significant move is the signing of France lock Yoann Maestri.
The 30-year-old Maestri, who has been capped 59 times by France, was originally going to join La Rochelle from Toulouse but had a change of heart after coach Patrice Collazo unexpectedly left the club last week.
According to French media reports, Stade Francais paid La Rochelle 700,000 euros ($825,000) in compensation.
Maestri won two Top 14 titles and the 2010 European Cup with Toulouse in a nine-year spell.
Parisian club Stade struggled this term, only avoiding relegation on the final day of the campaign, but will be expecting to rebuild under new coach Meyer.
The new acquisitions are a statement of intent from Stade after being bought last year by Swiss billionaire Hans-Peter Wild, who said last month he wanted the team to win the French title in the next "three or four years".