The transfer fee has not been disclosed but French media reported that PSG had paid Bayern a fee of 40 million euros ($43.87 million) plus add-ons for the player.
"I'm really excited! I've been waiting to join PSG for a long time, and it's finally happened. It's a very special day for me and I'm very happy to be here," Hernandez said in a statement.
Atletico Madrid, Hernandez's boyhood club, will also receive a significant fee, according to Spanish media reports.
The France international made 107 appearances for the German league champions since his arrival in 2019 but played only seven league games last season after suffering a cruciate ligament rupture injury at the World Cup that kept him sidelined for the remainder of the season.
Hernandez, PSG's fifth new signing, joins midfielders Manuel Ugarte and Lee Kang-in, central defender Milan Skriniar and forward Marco Asensio.
The club also sacked manager Christophe Galtier on Wednesday and replaced him with former Barcelona and Spain coach Luis Enrique.
