TP53 alterations are frequent relapse-acquired mutations in childhood acute lymphoblastic leukemia (ALL). The present study evaluated the clinical significance of relapsed childhood ALL in Taiwan. Diagnostic and/or relapsed bone marrow or peripheral blood was obtained from 111 children with relapsed ALL who were initially treated by using Taiwan Pediatric Oncology Group (TPOG) ALL protocols from January 1997 to May 2018. Mutations were detected by PCR and sequencing, as well as by multiplex ligation-dependent probe amplification to detect copy number alterations. Copy number and/or sequence alterations of TP53 were detected in 29% (28 of 98) and in 46% (6 of 13) of patients with relapsed B-cell and T-cell ALL, respectively. This incidence was much higher than that in several similar studies conducted in Caucasian populations. Seventy percent of all TP53 alterations were gained at relapse in 67 matched samples by back-tracking matched diagnostic samples. TP53 alterations were associated with lower 5-year event-free survival (EFS) and overall survival (OS) rates (P =.013 and P =.0002, respectively). Multivariate analysis confirmed the prognostic significance of TP53 alterations. Forty-five patients received hematopoietic stem-cell transplantations post-relapse. Patients with TP53 alterations (14/45) had inferior 5-year EFS and OS than patients without TP53 alterations after transplantation (P =.002 and P =.001, respectively). The significance of these TP53 alterations for patients who received transplantations was confirmed by multivariate analysis. In conclusion, TP53 alterations were enriched and useful as prognostic markers in relapsed childhood ALL.