A critical determinant of DNA repair pathway choice is the HORMA protein REV7, a small abundant adaptor which binds to various DNA repair proteins through its C-terminal seatbelt domain. The REV7 seatbelt binds to the REV3 polymerase to form the Polymerase ζ complex, a positive regulator of translesion synthesis (TLS) repair. Alternatively, the REV7 seatbelt binds to SHLD3 in the Shieldin complex, a positive regulator of NHEJ repair. Recent studies have identified another novel REV7 seatbelt-binding protein, CAMP (Chromosome Alignment-Maintaining Phosphoprotein), though its role in DNA repair is unknown. Here, we show that the REV7-CAMP complex promotes homologous recombination (HR) repair by sequestering REV7 from the Shieldin complex. CAMP competes directly with the SHLD3 subunit of the Shieldin complex for a limited pool of C-REV7, thereby inhibiting the REV7-mediated recruitment of the SHLD2 and SHLD1 effector subunits to DNA double strand breaks. CAMP thereby channels DNA repair away from error-prone NHEJ and towards the competing error-free HR pathway. Similarly, CAMP competes with the REV3 component of the POL-Zeta complex, thereby reducing the level of mutagenic TLS repair. CAMP has a distinct function in promoting chromosome alignment which is independent of its REV7 binding activity. Importantly, in human tumors, CAMP overexpression promotes HR, confers PARP inhibitor resistance, and correlates with poor prognosis. Thus, by binding to either REV3, SHLD3, or CAMP through its seatbelt, the REV7 protein can promote either TLS repair, NHEJ repair, or HR repair respectively.