Objective: Evidence has shown the importance of tumor necrosis factor alpha (TNF-alpha) in the pathophysiological feature in schizophrenia patients. This study aims to determine the impact of a single-nucleotide polymorphism (SNP) in the TNF-alpha gene promoter on the susceptibility, onset age, and cognitive function of schizophrenia. Method: The SNP -1031T>C in the TNF-alpha gene was genotyped in 905 patients and 571 healthy controls. The Positive and Negative Syndrome Scale (PANSS) was used to assess the schizophrenia symptoms and the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) for cognitive function. Results: There was no significant difference in allele or genotype distribution of the SNP -1031T>C between patients and controls (p = .85, p = .98). This polymorphism had no significant genotypic effect on the symptomatology assessed by the PANSS. Interestingly, this polymorphism was significantly correlated with onset age in schizophrenia patients (p = .004). We found an earlier onset age in patients with the TT genotype compared to those with the CT and CC genotypes (both p < .05). Moreover, there were significant genotypic effects on the immediate memory index, visuospatial/constructional index, and RBANS total score (all p < 0.05) in the patient group. Conclusions: Our results suggest that the SNP -1031T>C of the TNF-alpha gene may not be associated with susceptibility to schizophrenia but possibly acts as a modulator for its onset age as well as for cognitive deficits. General Scientific Summary The TNF alpha-1031T/C variant is not associated with schizophrenia or its clinical symptoms but with onset age and some cognitive deficits of schizophrenia.