It is imperative to explore potential biomarkers for predicting clinical outcome and developing targeted therapies for T-cell acute lymphoblastic leukemia (T-ALL). This study aimed to investigate the mutation patterns of tumor necrosis factor-alpha-inducing protein 3 (TNFAIP3, also known as A20) and its role in the prognosis of T-ALL patients. Polymerase chain reaction (PCR) and Sanger sequencing data from T-ALL (n= 49, JNU) and targeted sequencing data from T-ALL (n= 54, NFH) in our clinical center and a publicly available dataset (n= 121, PRJCA002270), were used to detect TNFAIP3 mutation. Three TNFAIP3 single nucleotide polymorphisms (SNPs; g.3033 C > T, g.3910 G > A, and g.3904 A > G) were detected in T-ALL in the JNU dataset, and g.3033 C > T accounted for the highest proportion, reaching 60% (6/10). Interestingly, TNFAIP3 mutation mainly occurred in adults but not pediatric patients in all three datasets (JNU, NFH, and PRJCA002270). T-ALL patients carrying a TNFAIP3 mutation were associated with a trend of poor overall survival (OS) (p= 0.092). Moreover, TNFAIP3 mutation was also an independent factor for OS for T-ALL patients (p= 0.008). Further subgroup analysis suggested that TNFAIP3 mutation predicted poor OS for T-ALL patients who underwent chemotherapy only (p< 0.001), and it was positively correlated with high risk and early T-cell precursor ALL (ETP-ALL) in two independent validation datasets (NFH and PRJCA002270). TNFAIP3 mutation mainly occurs in adult T-ALL patients, and it was associated with adverse clinical outcomes for T-ALL patients; thus, it might be a biomarker for prognostic stratification.