The 18-year-old forward arrives in South London as a free agent, having last played for Brisbane Roar in the A-League - the highest level of Australian football.
He made his professional debut for the Roar in October 2023 and went on to make 11 appearances in total during the 23/24 season, scoring once against Melbourne City and registering an assist against Adelaide United.
The youngster will now be a part of the Academy setup for the remainder of the campaign and in the coming seasons.
Upon signing, Brownlie said: “It feels amazing, absolutely amazing. The facilities here are incredible and I just want to get some minutes and start scoring goals.”
Academy Director Gary Issott added: “We were made aware of Rylan after seeing him play first-team football for Brisbane Roar.
“He’s an attacking player who can play anywhere along the front three and will feature as part of the Under-21s side.”