Rennie, a versatile 16-year-old forward capable of playing on either flank or through the middle, joins Fulham's academy after progressing through Celtic's youth ranks, having previously come through Edinburgh side The Spartans before being picked up by the Scottish champions.
He has already represented Scotland at both under-15 and under-16 level, scoring on his international debut in a 4-2 win over Slovakia in October 2024, and will link up with Andrew Joslin's under-18 squad for the coming season.
"It feels really good to sign for Fulham," Rennie told the club's official website. "It's good to get it done and I just want to get started and play my first games.
"A London club, with the history of Fulham, with the reputation of the academy, it was a no-brainer to sign here. I'm an attacking player. My first thought is always to beat my man, use my pace, get balls in the box, attack, attack, attack."
At the other end of the club's pathway, midfielder Luke Harris has joined League One side Wigan Athletic on a season-long loan, becoming the third player to arrive at the DW Stadium from a Premier League club this week, following Reggie Walsh and Charlie Gray's loan moves from Chelsea and Manchester City respectively on Wednesday.
The 21-year-old has already had loan experience across the EFL, with spells at Exeter City, Birmingham City, Oxford United and Wycombe Wanderers, and could make his Wigan debut against Leyton Orient on Saturday.
He will be reunited with Latics boss Gary Caldwell, who previously worked with Harris during his time on loan at Exeter. Harris is yet to establish himself in Fulham's first-team picture since coming through the club's academy, and a fourth loan move in as many seasons is intended to give him a sustained run of senior football.
Between them, the two moves illustrate the breadth of Fulham's academy operation, from recruiting one of Scottish football's brightest teenage prospects to sending a more experienced product of the system out for regular senior football.