Paris St.-Germain, the French soccer powerhouse bankrolled by the state of Qatar, has signed Lionel Messi, the Argentine star who is one of the best players of his or any generation.

The agreement, for two seasons plus an option year, comes days after Messi bid a tearful farewell to F.C. Barcelona, the club where he had spent his entire professional career. It concluded a brief and exclusive bidding war for Messi and added unmatched star power to the roster of P.S.G., a superteam that has become a fixture in the late stages of Europe’s Champions League, the world’s richest soccer competition.