Ben, the brother of Sea Eagles and Origin stars Tom and Jake Tbrojevic, has joined the Sea Eagles on a two-year deal.
The centre/back-rower has come through the Sea Eagles' Harold Mathews and SG Ball junior representative teams and has played in the Jersey Flegg competition this season.
Tbrojevic also represented NSW in the U16 and U18 teams as well as playing for Mona Vale Raiders Club.
Manly coach Des Hasler told seaeagles.com.au that he is excited about the 18-year-old prospect.
“Ben is a very talented young player who has done well coming through our junior ranks,’’ Hasler said.
“There is no doubt he has plenty of ability and it’s now a matter of further developing his game and bringing him through slowly as he steps up to the next level.”