The midfielder joins Palace after helping Airdrieonians reach the Scottish Championship.
Devenny joined Airdrieonians permanently in the summer of 2022 after playing for Kilmarnock.
Over two seasons, he scored five goals and seven assists in 54 Diamonds appearances. Devenny scored in the League One play-off final penalty shootout against Hamilton Academical in 2022/23, playing 41 times and winning promotion.
“I’m absolutely delighted to be here at Crystal Palace,” Devenny said after signing. It’s a major change, but I’m excited to start. I’m excited about the wonderful facilities.”
Academy Director Gary Issott said: “Justin was someone we were aware of last season after he established himself in the promotion-winning Airdrieonians team.
“He has played first-team football from an early age and joins our exciting Under-21s squad to provide midfield competition.”