Fellaini, 36, had been playing in China with Shandong Taishan since leaving Old Trafford in February 2019.
He spent six seasons at United, playing 177 games and scoring 22 goals for the Red Devils. During his time at the club, he worked under David Moyes, Louis van Gaal and Jose Mourinho.
Fellaini was the first signing of United’s post-Sir Alex Ferguson era, with Moyes paying £27.5 million to sign him from former club Everton on deadline day in September 2013.
Fellaini won trophies at Old Trafford
He was a much-maligned figure during his first season at the club but gradually won over United fans with his workrate and his goalscoring touch.
United won the FA Cup, the Europa League and the EFL Cup – among other minor honours – throughout Fellaini’s stay, and he played in all three winning finals.
For Belgium, he won 87 caps, scoring 12 goals and featured at the World Cups in 2014 and 2018 as well as Euro 2016.
“I am writing this post to announce that after a career of 18 years, I am retiring from professional football,” he wrote on Instagram.
“But it all started long before these 18 years. I remember, like the day of yesterday, my first game as a child, the endless training sessions with my dad, my first selection for Standard de Liege, and my first selection for the Belgian national team.
Time flies! What an incredible journey it has been!
“I am so grateful to have played the sport I love at the highest level. I am proud to have represented Standard in Belgium, Everton and Manchester United in the UK, and Shandong Taishan in China.
“It was an honor to have played for the Belgian national team at the Olympics of 2008, Euro 2016 and the World Cups of 2014 and 2018.
“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.
“Your encouragement and passion have been a constant source of inspiration for me. All of this would have been impossible without my family, teammates, coaches, advisers and everyone who has been a part of my journey.
“Your love, guidance, support, and friendship have been invaluable. Thank you! 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. See you soon!”
Following Fellaini’s announcement, United’s stars of the recent past were quick to pay tribute.
Ex-goalkeeper David de Gea said: “Enjoy life brother.” Ander Herrera wrote: “Congratulations amigo and all the best for your next chapter.”
Former United stalwart Javier Hernandez declared: “What a career brother!” and Jesse Lingard said: “What a guy big bro.”
Eric Bailly and Phil Jones were also full of praise for their former team mate, with Bailly saying: “Congratulations on your career my brother,” and Jones offering: “Congratulations Felli.”