Oyedele, 19, has scored five times and registered one assist in 41 appearances for the Red Devils at Under-18s level, while he has also played 23 times for the Under-23s.
The midfielder was involved in Man United's preparations for the 2024-25 campaign, but the club have now confirmed that he has departed in search of pastures new.
"Maxi Oyedele has completed a permanent move from Manchester United to Legia Warsaw," read a statement on the club's official website.
Oyedele posts Man United tribute on social media
"My time at Manchester United has finally come to end. I want to say thank you to the only club I've ever known. Honoured to say I've been at the club for 12 years and had some unbelievable experiences with some amazing teammates," Oyedele posted on his official Instagram account.
"Thank you to all the coaches, staff and the fans, I will never forget the memories I have made at this club. I wish everyone at Manchester United all the best as I leave ready to start the next chapter of my career. This club will always be my home and once again thank you. Once a red, always a red."
Oyedele provided the assist for Joe Hugill to score against Rangers during this summer's friendlies, and he has previously spent time out on loan at Altrincham and Forest Green Rovers.
However, his departure on a permanent basis has now been confirmed.
Oyedele "very excited" about Legia move
"I feel like I was supposed to join Legia for a long time, so I'm glad this transfer has come to fruition," Sportowe Fakty quotes him as saying. "I'm very excited about playing on this pitch.
"Of course, I'm aware of what a great club Legia is. I want to win the championship with the club and be part of this team. For me, it's a huge chance to earn my name.
"When I came here, I had a talk with the coach. He presented me with a detailed and extensive plan (of how they are going to use me)."
Oyedele could potentially make his debut for his new club in Sunday's league clash against Slask Wroclaw.