The club made the announcement on the social media site.
Nani joined United from Sporting in 2007 and, during his time at Old Trafford, he won four English Premier League titles and a UEFA Champions League crown.
He signed a five-year deal in September 2013, but struggled to hold down a first-team place under David Moyes and Louis van Gaal, who took over as coach from Moyes. It was agreed he be sent out on loan to his former club.
Reports said a transfer fee for the 28 year-old was being agreed in the region of $8.7 million.