TET1/2/3 are methylcytosine dioxygenases that regulate cytosine hydroxymethylation. Tet1/2 are abundantly expressed in HSC/HPCs and are implicated in hematological malignancies. Tet2 deletion in mice causes myeloid malignancies, while Tet1-null mice develop B cell lymphoma after an extended period of latency. Interestingly, TET1/2 are often concomitantly downregulated in acute B-lymphocytic leukemia. Here, we investigated the overlapping and non-redundant functions of Tet1/2 using Tet1/2 double-knockout (DKO) mice. DKO and Tet2-/- HSC/HPCs show overlapping and unique 5hmC and 5mC profiles. DKO mice exhibit strikingly decreased incidence and delayed onset of myeloid malignancies in comparison to Tet2-/- mice and in contrast develop lethal B cell malignancies. Transcriptome analysis of DKO tumors reveals expression changes in many genes dysregulated in human B cell malignancies, including LMO2, BCL6, and MYC. These results highlight the critical roles of TET1/2 individually and together in the pathogenesis of hematological malignancies.