Harper was today released by the Bulldogs, freeing him to immediately join Manly.
The 21-year old has played two NRL games after making his debut in Round 25 of last season.
“We have been looking at Morgan for quite some time. He is an exciting, fast and elusive centre and will complement the Manly squad,” Sea Eagles Recruitment Officer Scott Fulton told the club website.
“We are very happy to have secured his services.”
Harper represented the Junior Kiwis in 2018.