Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD+ salvage pathway and a promising therapeutic target in cancer. Resistance to NAMPT inhibitors, such as FK866, remains a key limitation to their clinical translation. While acquired resistance in cancer cell lines has been linked to target mutations, increased drug efflux, and metabolic reprogramming, innate resistance mechanisms have been poorly studied. Addressing this gap is crucial for identifying patient subgroups that are most likely to benefit from NAMPT-targeted therapies. Advanced castration resistance prostate cancer (CRPC) lacks effective targeted treatments. Among its heterogeneous subtypes, stem cell-like CRPC (CRPC-SCL) is characterized by independence from androgen receptor (AR) signaling, dependency on YAP/TAZ, and mesenchymal traits. In this study, we identify the YAP/nicotinamide N-methyltransferase (NNMT) axis as a key regulator of innate sensitivity to FK866 in stem-like mesenchymal CRPC cells. Using genetic and pharmacological models, we show that YAP or NNMT silencing rescues PC3 cells from FK866-induced apoptosis, endoplasmic reticulum stress, and NAD(H) depletion. Metabolomic profiling confirmed that NNMT activity depletes nicotinamide, sensitizing cells to FK866. We further validated NNMT upregulation across clinical CRPC-SCL datasets, where it strongly correlates with mesenchymal and therapy-resistant phenotypes. Murine prostate cancer cells with mesenchymal/stemness phenotypes (DVL3-SCM), that exhibit NNMT overexpression and high aggressiveness in vivo, also show increased sensitivity to FK866 compared with their parental counterparts (DVL3-PAR). In conclusion, we identify the YAP/NNMT axis as a determinant of innate sensitivity to NAMPT inhibition in prostate cancer. These findings support the use of NNMT as a predictive biomarker for NAD+-targeting therapies and provide mechanistic insight into a metabolic vulnerability of the CRPC-SCL subtype. Targeting the YAP/NNMT/NAMPT axis may represent a novel strategy for treating stem-like/mesenchymal, therapy-resistant prostate cancers.