The Argentina midfielder was wanted by a host of clubs but has joined the Buenos Aires outfit on a one-year deal after leaving Italian side Lecce
Los Azulgrana fought off the challenge of several other clubs including Boca Juniors, River Plate, Racing Club and Independiente who had all registered the player as part of their squad in the hopes of sealing a deal for the 27 year-old.
Marcelo Tinelli, who was recently appointed member of San Lorenzo's Executive Committee, was instrumental in securing the Argentinian's signature.
Piatti had unsuccessful spells at Roma and Galatasaray before enjoying stints at Chacarita Juniors, Saint Etienne, Gimnasia LP, Independiente and Lecce.
Speaking to the club's official website, the midfielder expressed his delight at signing for San Lorenzo: "I thank the Club. I'm happy to be in [here]. San Lorenzo is a great club and I will give my best."
"I arrive at San Lorenzo at a very good time," he continued. "I really wanted to return to [Argentina] to play. I will prove that whole-heartedly. I will try to find a place in the team and try my best for San Lorenzo... I saw the proposal of San Lorenzo, the players they have and really liked it."
San Lorenzo also announced the signing of Uruguayan attacking midfielder Matias Mirabaje from Montevideo Wanderers. Both players will be pushing for a start against Estudiantes on Sunday.