The 17-year-old has completed his move across the city and will join Manchester United's Under-18 squad, the club confirmed on their official website.
Cassim was part of the Manchester City squad that beat United 2-1 to win the FA Youth Cup in May, a fixture that marked the second all-Manchester final in the competition's history and continued Manchester City's recent dominance of the derby at youth level, having also won the trophy in 2020 and 2024.
The versatile midfielder, capable of playing across United's engine room from a holding role through to a more advanced number ten position, scored three goals in City's Under-18 Premier League side last season, having broken into the age group while still only 15 years old.
He has also represented England at Under-17 level, adding international pedigree to a deal that forms part of Manchester United's wider push to strengthen their academy under the club's revamped football operations led by INEOS.
Cassim's arrival follows the same route taken by fellow teenager David Eze, another Manchester City academy midfielder who completed his own switch to Old Trafford in the same transfer window this summer, with the pair having been City team-mates at Under-18 level.
It is not the first time United have raided their neighbours' academy for emerging talent. The club previously signed twins Jack and Tyler Fletcher from City for a reported £1.25 million in 2023, underlining a pattern of United targeting City-developed prospects in recent years.
Manchester City's academy has produced a string of England youth internationals in recent seasons, making the loss of players such as Cassim a source of frustration at the Etihad Campus even as the club continues to enjoy success at youth level domestically.
Cassim will now begin life in United's youth system, with the Old Trafford club hopeful he can follow the pathway into senior football that a growing number of academy graduates have taken in recent seasons.
United's academy has increasingly become a focal point of the club's rebuilding project, with the recruitment of promising teenagers from rival clubs seen as a long-term investment.