Pato, 31, had also been linked to Inter Miami and Iraq club Al Diwaniya.
But Florida rivals City swooped ahead of David Beckham's franchise to seal a one-year deal on a free.
Orlando City supremo Luiz Muzzi said: “We are very excited to welcome Pato to Orlando. He's a veteran player who has featured at some of the top clubs around the world and has a proven track record at the international level.
"The experience that Pato brings with him will be invaluable to the Club.
“Pato is a high-calibre player and we're excited to add another great target up top."