"Paris Saint-Germain is delighted to announce the signing of Leo Messi," the club said.
"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 Saint-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."
The Argentina international, who scored 672 goals in 778 appearances for the Catalan giants, left for France on Tuesday.
That was two days after he said a tearful farewell to the club he had helped to win 35 trophies after they were unable to agree on a contract extension that satisfied La Liga's financial regulations.
"I am excited to begin a new chapter of my career at Paris Saint-Germain," Messi said.
"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 on to the pitch at the Parc des Princes."
The Qatar Sports Investments-backed club, managed by Mauricio Pochettino, stepped up its quest for Champions League glory in spectacular style.
"I am delighted that Lionel Messi has chosen to join Paris Saint-Germain and we are proud to welcome him and his family to Paris," said club president Nasser Al Khelaifi.
"He has made no secret of his desire to continue competing at the very highest level and winning trophies, and naturally our ambition as a club is to do the same.
"The addition of Leo to our world-class squad continues a very strategic and successful transfer window for the club.
"Led by our outstanding coach and his staff, I look forward to the team making history together for our fans all around the world."