Bullemor was released by the Broncos last month amid speculation Manly would be the club to secure the prop's signature.
It means Bullemor will need to impress coach Des Hasler from the word go if he is to slot into the first grade starting team.
The contract for Bullemor is two years in length, meaning he will stay at the club until the end of the 2023 season, and also marks the Sea Eagles' first signing from outside the club for 2022.
Manly this morning announced all of Tevita Funa, Jack Gosiewski, Zac Saddler and Moses Suli were leaving the club, alongside Curtis Sironen, while they also re-signed Jorge Taufua and Morgan Boyle in what has been a busy day for the Sea Eagles as they shape their 2022 roster.
The 21-year-old Bullemor has 25 matches of NRL experience under his belt and Manly coach Des Hasler said he would be a good addition to their forward pack.
“Ethan is a very highly promising player who will add another dimension to our strong forward pack," Hasler said.
“He is only young and still has plenty to learn but Ethan certainly has the talent and ability to succeed.”