RAF kinase inhibitors are clinically active in patients with BRAF (V600E) mutant melanoma. However, rarely do tumors regress completely, with the majority of responses being short-lived. This is partially mediated through the loss of negative feedback loops after MAPK inhibition and reactivation of upstream signaling. Using a genome shRNA screen to target all known deubiquitinating (DUB) enzymes we identify USP28 as a novel regulatory of MAPK activity. Furthermore, we demonstrate that USP28 functions through a feedback loop to destabilize RAF family members. Loss of USP28 stabilizes BRAF enhancing downstream MAPK activation and promotes resistance to RAF inhibitor therapy in culture and in vivo models. Importantly, we demonstrate that USP28 is deleted in a proportion of melanoma patients and may act as a biomarker for response to BRAF inhibitor therapy in patients. To identify novel therapeutic strategies for melanoma patients harbouring USP28 deletions we performed a chemical compound screen and identify rigosertib as being synthetically lethal with USP28 loss. Taken together our results show that loss of USP28 enhances MAPK activity through the stabilization of RAF family members and is a key factor in BRAF inhibitor resistance.