The 34-year-old became a free agent last month after ultimately failed negotiations over extending his Barcelona contract failed, and PSG moved quickly to snap him out.
A statement from the club read: “The six-time Ballon d’Or winner is justifiably considered a legend of the game and a true inspiration for those of all ages inside and outside football.
“The signing of Leo reinforces Paris St Germain’s aspirations as well as providing the club’s loyal fans with not only an exceptionally talented squad, but also moments of incredible football in the coming years.”
PSG’s main aim is to win the Champions League and Messi’s addition has made them favourites as they look to go one better than 2020, where they lost in the final to Bayern Munich.
Messi’s own ambitions to again win the Champions League are well known and the world’s greatest player is aiming to win the game’s biggest prize at his new club.
He said: “I am excited to begin a new chapter of my career at Paris Saint-Germain. Everything about the club matches my football ambitions. I know how talented the squad and the coaching staff are here.
“I am determined to help build something special for the club and the fans, and I am looking forward to stepping out onto the pitch at the Parc des Princes.”