The former Brazil international agreed personal terms after completing a medical in Rome, where Tianjin were undergoing pre-season training.
Pato, 27, who joins coach Fabio Cannavaro and Belgium international Axel Witsel at the newly-promoted club, posted a photo of himself and the former Italian international online to confirm news of the transfer.
"I'm very happy to be a part of this new family at Tianjin Quanjian," Pato said in the post.
The former child prodigy, who signed for Milan as a 16-year old, has failed to live up to expectations and has enjoyed patchy success in his career. He joined Chelsea on loan during the second half of last season, but struggled to make an impact.
Pato only joined Villarreal in July last year from Corinthians and has been inconsistent, making just 11 appearances in La Liga and netting twice for the Spanish club.