Kyle de Silva grabbed the only goal of the game as Palace's under-21 side beat Millwall thanks to a last-minute strike.
The attacking midfielder grabbed his second goal of the season with a deflected effort in stoppage time that ensured Gary Issott's side grabbed all three points against their local rivals.
In a game of few chances, Palace just about edged out the Lions on their own patch, and the result is a welcome boost to Issott's side who had lost their previous two games and keeps their hopes of winning a play-off spot in tact.
Millwall: King, Siafa, Hoyte, Nelson, Wood, Webb (Bryon, 90mins), Powell, O'Brien, Pavey (Philpot, 80mins), Onyedinma (Newby, 46mins), Marquis. Subs not used: Girling, Parr.
Palace: Kettings, Egbo, George, Chambers, Croll (Magri 70), Boateng, Black, Gray, De Silva, Allassani (Berkley-Agyepong 55), Ferrier (Adarabioyo 55). Subs not used: Gregory, Sow.