The 20-year-old put pen to paper on the deal on Monday, and will be available for selection for Saturday's Sky Bet League Two opener against Colchester United at Sixfields.
Hailing from Glasgow, Leonard began his career in the Rangers youth system before moving to Heart Of Midlothian.
He made the long move south to join the Brighton Academy in 2018, and last season made his senior debut for Graham Potter’s side as he started in a 2-0 Carabao Cup win at Cardiff City.
Leonard went on to make a second start in the second round win over Swansea City, and then also played in the Seagulls Under-21s' Papa John EFL Trophy group matches - captaining the side in their 2-1 win against the Cobblers at Sixfields.
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He also played in 22 of the Brighton Under-23s’ Premier League 2 campaign last season, scoring four goals.
The midfielder has been capped throughout the age levels by Scotland, and last year he made his debut for the national side's Under-21s, going on to make seven appearances for Scot Gemmill’s side last season.