Portugal international Nani arrives in Orlando as a designated player on a free transfer from Portuguese side Sporting CP.
The 32-year-old, who won four Premier League titles and the Champions League during his time at United, only re-joined boyhood club Sporting for a third spell in July.
"This is an exciting day for our organisation," said Orlando executive vice-president of soccer operations Luiz Muzzi. "Nani brings tremendous experience to our roster.
"He's a dynamic winger with quality abilities to move and cross the ball, impact games and lead the offensive play."
Nani has struggled to settle since leaving United in 2015, spending time at Sporting, Turkish giants Fenerbahce, LaLiga side Valencia and Serie A outfit Lazio.
The veteran has amassed 112 international appearances for Portugal, while he was part of the nation's triumphant Euro 2016 campaign.
Nani will add some star quality to Orlando, who have not reached the MLS playoffs since joining the league in 2015, after Kaka's departure in 2017.