Fanconi anemia (FA) is a chromosome instability syndrome of children caused by inherited mutations in one of FA genes, which together constitutes a DNA interstrand cross-link (ICL) repair, or the FA pathway. Monoubiquitination of Fanconi anemia group D2 protein (FANCD2) by the multi-subunit ubiquitin E3 ligase, the FA core complex, is an obligate step in activation of the FA pathway, and its activity needs to be tightly regulated. FAAP20 is a key structural component of the FA core complex, and regulated proteolysis of FAAP20 mediated by prolyl cis-trans isomerization and phosphorylation at a consensus phosphodegron motif is essential for preserving the integrity of the FA core complex, and thus FANCD2 monoubiquitination. However, how ubiquitin-dependent FAAP20 degradation is modulated to fine-tune FA pathway activation remains largely unknown. Here, we present evidence that FAAP20 is acetylated by the acetyltransferase p300/CBP on lysine 152, the key residue which when polyubiquitinated results in the degradation of FAAP20. Acetylation or mutation of the lysine residue stabilizes FAAP20 by preventing its ubiquitination, thereby protecting it from proteasome-dependent FAAP20 degradation. Consequently, disruption of the FAAP20 acetylation pathway impairs FANCD2 activation. Together, our study reveals a competition mechanism between ubiquitination and acetylation of a common lysine residue that controls FAAP20 stability and highlights a complex balancing between different posttranslational modifications as a way to refine the FA pathway signaling required for DNA ICL repair and genome stability.