A pediatric patient diagnosed initially with B‐lymphoblastic leukemia (B‐ALL) relapsed with lineage switch to acute myeloid leukemia (AML) after chimeric antigen receptor T‐cell (CAR‐T) therapy and hematopoietic stem cell transplant. A TCF3‐ZNF384 fusion was identified at diagnosis, persisted through B‐ALL relapse, and was also present in the AML relapse cell population. ZNF384‐rearrangements define a molecular subtype of B‐ALL characterized by a pro‐B‐cell immunophenotype; furthermore, ZNF384‐rearrangements are prevalent in mixed‐phenotype acute leukemias. Lineage switch following CAR‐T therapy has been described in patients with KMT2A (mixed lineage leukemia) rearrangements, but not previously in any patient with ZNF384 fusion.