Cancer cells exploit the expression of the programmed death-1 (PD-1) ligand 1 (PD-L1) to subvert T-cell mediated immunosurveillance. Therapies that disrupt PD-L1 mediated tumour immune evasion have achieved substantial clinical success, highlighting the need to understand the molecular regulation of PD-L1 expression. Using a genome-wide CRISPR/Cas9 loss of function screen we have identified the uncharacterized protein CMTM6 to be a key regulator of both constitutive and cytokine-induced PD-L1 expression in a broad range of cancer cells. CMTM6 is a previously uncharacterised, ubiquitously expressed protein that associates with PD-L1 in a membrane-anchored complex. CMTM6 is not required for trafficking of PD-L1 to the cell surface but is essential to maintain PD-L1 expression at the plasma membrane, and in its absence PD-L1 is targeted for lysosome-dependent degradation. Using a quantitative global proteomics approach to profile plasma membrane proteins regulated by CMTM6, we find that CMTM6 displays remarkable specificity for PD-L1. Importantly, loss of CMTM6 specifically decreases PD-L1 without compromising the cell surface expression or antigen presentation capacity of MHC class I. CMTM6 depletion in tumour cell lines alleviates PD-L1-mediated suppression of tumour specific cytotoxic T-cell activity both in vitro and in vivo, highlighting the functional importance of CMTM6 in maintaining the PD-L1/PD-1 immune checkpoint. Together these findings provide novel insights into the biology of PD-L1 regulation, identify a new master regulator of this critical immune checkpoint and reveal a potential therapeutic target to overcome immune evasion by cancer cells.