An inability to replicate the genome can cause replication stress and genome instability. Here, we develop BLOCK-ID, a proteomic method to identify and visualize proteins at stressed replication forks. This approach successfully identified novel mediators of the replication stress response, including the chromatin acetylation reader protein TRIM24. We uncovered a crucial role for TRIM24 in coordinating Alternative Lengthening of Telomeres (ALT), a replication stress-directed telomere extension mechanism. Our data reveal that TRIM24 is recruited to telomeres via a p300/CBP-dependent acetylation chromatin signaling cascade to organize the assembly of ALT-associated PML bodies (APBs) and promote de novo telomere DNA synthesis. Unexpectedly, tethering of TRIM24 at telomeres was sufficient to stimulate de novo telomere DNA synthesis in a SUMO-dependent, but p300/CBP- and PML-independent manner. Collectively, these findings uncover an indispensable epigenetic signaling pathway involving TRIM24 and p300/CBP that mediate ALT telomere maintenance.