The midfielder joins Palace after impressing for Airdrieonians in their recent promotion to the Scottish Championship, the second-tier of Scottish football.
Devenny came through the ranks at Kilmarnock before moving to Airdrieonians on a permanent basis in the summer of 2022.
He made 54 appearances for the Diamonds over two seasons, notching five goals and seven assists. During their 2022/23 promotion-winning campaign, Devenny played 41 times, scoring in the penalty shootout of the League One play-off final against Hamilton Academical.
Upon signing, Devenny said: “I’m absolutely delighted to be here at Crystal Palace. It’s a big step up, but I can't wait to get started. The facilities are amazing and I’m really looking forward to it.”
Academy Director Gary Issott added: “Justin was someone we were made aware of last season after he established himself in the promotion-winning team at Airdrieonians.
“He has played first-team football from an early age, and joins our exciting Under-21s squad to provide competition in the midfield area.”