Alzate will now joint up with the Seagulls under-23 squad.
Alzate, who can play on either wing or upfront, joins Brighton on a three-year contract for an undisclosed fee.
Under-23s coach Simon Rusk told his club`s official website: “We are delighted to welcome Steven into the squad.
“He is someone who did well to break into the Leyton Orient first-team from their academy last season, and adds a new dimension to our attacking options.”
The 18-year-old leaves Orient after joining the club at under-13 level. He made a dozen appearances in League Two last season.
(Photo: Leyton Orient FC)