Fellaini, who plays for Chinese club Shandong Luneng, told his followers on Twitter that he tested positive for the infectious disease, saying he hopes to return to the pitch "as soon as possible" after overcoming the illness.
Shandong Luneng announced that Fellaini arrived back in China on Friday and tested positive after being placed in 14-day self-isolation.
"The official test confirmed that Fellaini tested positive for the coronavirus during his quarantine observation period," a statement from the Chinese club read, courtesy of ESPN FC. "The player's body temperature has been normal and he is comfortable. He has received further treatment at a designated medical institution.
"The club will do its best to cooperate and provide help for the player's treatment and rehabilitation, and wishes for his swift recovery. Fellaini has expressed his gratitude to the fans and his friends at the club, adding that he will get better soon and be stronger than ever when he returns."
Fellaini started his 11-year journey in the Premier League with Everton in 2008 before moving to United five years later. After six years at Old Trafford, Fellaini agreed to join Shandong Luneng on a five-year contract in January 2019.
He's the latest high-profile player to be diagnosed with COVID-19, joining footballers such as Paulo Dybala, Blaise Matuidi, Daniele Rugani, and Callum Hudson-Odoi.
Mikel Arteta, Fellaini's former Everton teammate and the current Arsenal manager, also tested positive for the coronavirus earlier this month.