BACKGROUND: Natural killer (NK) cells' unique ability to kill transformed cells expressing stress ligands or lacking major histocompatibility complexes (MHC) has prompted their development for immunotherapy. However, NK cells have demonstrated only moderate responses against cancer in clinical trials. METHODS: Advanced genome engineering may thus be used to unlock their full potential. Multiplex genome editing with CRISPR/Cas9 base editors (BEs) has been used to enhance T cell function and has already entered clinical trials but has not been reported in human NK cells. Here, we report the first application of BE in primary NK cells to achieve both loss-of-function and gain-of-function mutations. RESULTS: We observed highly efficient single and multiplex base editing, resulting in significantly enhanced NK cell function in vitro and in vivo. Next, we combined multiplex BE with non-viral TcBuster transposon-based integration to generate interleukin-15 armored CD19 chimeric antigen receptor (CAR)-NK cells with significantly improved functionality in a highly suppressive model of Burkitt's lymphoma both in vitro and in vivo. CONCLUSIONS: The use of concomitant non-viral transposon engineering with multiplex base editing thus represents a highly versatile and efficient platform to generate CAR-NK products for cell-based immunotherapy and affords the flexibility to tailor multiple gene edits to maximize the effectiveness of the therapy for the cancer type being treated.