Juvenile Myelomonocytic Leukemia (JMML), Acute Myeloid Leukemia (AML) and other myeloproliferative neoplasms (MPN) are genetically heterogeneous but frequently display activating mutations in Ras GTPases and expression of Signal Transducer and Activator of Transcription 3 (STAT3). Altered STAT3 activity is observed in up to 50% of AML correlating with poor prognosis. Activated STAT proteins, classically associated with tyrosine phosphorylation, support tumor development as transcription factors, but alternative STAT functions independent of tyrosine phosphorylation have been documented, including a role for serine phosphorylated STAT3 in mitochondria supporting transformation by N- and K-Ras. We examined the requirement for STAT3 in experimental K-Ras-dependent hematopoietic neoplasia in mice. We show that STAT3 is phosphorylated on S727 but not Y705 in diseased animals. Moreover, a mouse with a point mutation abrogating STAT3 S727 phosphorylation displayed delayed onset and decreased disease severity with significantly extended survival. Activated K-Ras required STAT3 for cytokine-independent growth of myeloid progenitors in vitro, and mitochondrially restricted STAT3 and STAT3-Y705F, both transcriptionally inert mutants, supported factor-independent growth. STAT3 was dispensable for growth of normal or K-Ras-mutant myeloid progenitors in response to cytokine stimulation. However, abrogation of STAT3-S727 phosphorylation impaired factor-independent malignant growth. These data document that serine phosphorylated mitochondrial STAT3 supports neoplastic hematopoietic cell growth induced by K-Ras.