Background: NOS3 (endothelial NOS, eNOS) is a member of the nitric oxide synthase (NOS) enzyme family, mainly participating in nitric oxide (NO) generation. NOS3 has been reported to inhibit apoptosis and promote angiogenesis, proliferation, and invasiveness. However, the expression pattern of NOS3 and its diagnostic and prognostic potential has not been investigated in a pan-cancer perspective. Methods: In this research, data from the Genotype-Tissue Expression (GTEx), the Cancer Genome Atlas (TCGA), the Cancer Cell Line Encyclopedia (CCLE) and the Cancer Therapeutics Response Portal (CTRP) were employed and NOS3 expression was comprehensively analysed in normal tissues, cancer tissues and cell lines. Its relationship with clinical phenotypes and drug responses was also analysed. Results: In normal tissues, NOS3 was expressed at the highest levels in the spleen and the lowest levels in the blood. Compared with the corresponding normal tissues, its expression in tumour was significantly increased in seven tumour types but decreased in eight tumour types. And NOS3 expression was positively or negatively related to tumour stage and overall survival of patients depending on the tumour types. The expression of NOS3 was related to the response to ‘SR8278’. Conclusions: NOS3 was differentially expressed in tumour tissues, and had prognosis value in some tumour types. And its correlation to drug response warrants further investigation.