Martial, 28, left Old Trafford in June to bring his nine-year stay at the 13-time Premier League champions to an end.
He remained a free agent for three months, but is now set to return to the pitch after signing a contract with a European giant.
The Frenchman joined Greek club AEK Athens on Wednesday after being handed the biggest contract in the club's history. He has signed a three-year deal that will earn him £7.5m including bonuses, and revealed he can't wait to get started.
'I am very happy,' Martial said following confirmation of his move. 'AEK is an opportunity for me and I will give my best to achieve things, win trophies and bring joy to the world of the team.'
Former Man United striker Anthony Martial has joined AEK Athens on a three-year contract
Martial was made to feel welcome straight away at AEK after being mobbed by 2,000 fans at a Greek airport when he touched down in the country on Wednesday night.
AEK have won 13 Greek titles in their history, but finished second in the league last term, and will hope Martial can help them return to the top of the table this time around.
Martial has a point to prove following a disappointing end to his time at United that saw him score just one league goal in 13 outings last season.
Injuries and inconsistent form hampered his final years at the club, and he was sent out on loan to Sevilla for the second half of the 2022-23 season.
He finished with 90 goals in 317 appearances for United, and left the club three months ago after failing to earn a contract extension.
Martial joins a club who have made a strong start to the season, having taken 10 points from their opening four league matches.
They are back in action on Sunday, and Martial could be pushing for his first minutes for his new club.
However, his fitness may be a concern as he has not played at all since December due to illness, injury and falling out of favour with Erik ten Hag at United.