The risk stratification and final outcomes in patients with nasopharyngeal carcinomas (NPC) still remain suboptimal. Our principal goals were to identify and validate targetable metabolic drivers relevant to pathogenesis of NPC using a published transcriptome. One prominently downregulated gene regulating amino acid metabolism was found to be argininosuccinate synthetase (ASS1). Attributable to epigenetic DNA methylation, ASS1 deficiency may link to the therapeutic sensitivity to the arginine-depriving agents and promote tumor aggressiveness through its newly identified tumor suppressor function. ASS1 immunohistochemistry was therefore examined in a well-defined cohort of 124 NPC biopsy specimens and in the neck lymph node metastases of another ten independent cases. For the latter, bisulphite pyrosequencing was performed to evaluate the extent of ASS1 gene methylation. ASS1 protein deficiency was identified in 64 of 124 cases (51.6%), significantly related to T3-T4 status (p = 0.006), and univariately associated with inferior local recurrence-free survival (p = 0.0427), distant metastasis-free survival (DMFS; p = 0.0036), and disease-specific survival (DSS; p = 0.0069). Together with advanced AJCC stages III-IV, ASS1 protein deficiency was also independently predictive of worse outcomes for the DFMS (p = 0.010, hazard ratio = 2.241) and DSS (p = 0.020, hazard ratio = 1.900). ASS1 promoter hypermethylation was detected in eight of ten neck nodal metastatic lesions by bisulphite pyrosequencing and associated with ASS1 protein deficiency (p < 0.001). In summary, ASS1 protein deficiency was seen in approximately a half of NPCs and associated with advanced T classification, DNA methylation, and clinical aggressiveness, consistent with its tumor suppressor role. This aberration may render pegylated arginine deiminase as a promising strategy for ASS1-deficient NPCs.