"I am writing this post to announce that after a career of 18 years, I am retiring from professional football," the 36-year-old wrote on Instagram.
The midfielder made his professional debut for Standard Liege in 2006 before moving two years later to Everton for 20 million euros ($21 million), becoming at the time the most expensive player in the history of Belgian football.
A cult hero with Toffees fans due to his aerial prowess and eye-catching afro haircut, Fellaini followed David Moyes to United when the former Everton boss took charge at Old Trafford in 2013.
He won the 2016 FA Cup and Europa League and League Cup - both in 2017 - during his time with United, which included 177 appearances and 22 goals.
In 2019, Fellaini signed for Shandong Taishan in China, but did not extend his contract last November. His last match was the Chinese Cup final which Shandong lost to Shanghai Shenhua.
Fellaini won 87 caps for Belgium, scoring 18 goals, and helping them finish third at the 2018 World Cup before enjoying league and cup success with Shandong.
"What an incredible journey it has been! I am so grateful to have played the sport I love at the highest level," said Fellaini, who also featured at the 2014 World Cup and Euro 2016.
"I am proud to have represented Standard in Belgium, Everton and Manchester United in the UK, and Shandong Taishan in China. I have played in front of the best fans in the world, and I want to take this moment to thank all of them for their unwavering support throughout my career.
"As I embark on this new chapter in my life, I look forward to new challenges, but be assured, I will continue to support the beautiful game in any way I can," he wrote.