Internal tandem duplications of fms-like tyrosine kinase 3 (FLT3-ITD) in acute myeloid leukemia (AML) contribute to a significantly poorer prognosis by causing constitutive activation of the FLT3 receptor and producing aberrant signaling through signal transducer and activator of transcription 5 (STAT5). STAT5 signaling in this AML subtype and drives cell survival and proliferation. Additionally, our group has previously found that STAT5 signaling directly contributes to the genomic instability of FLT3-ITD AML, making these cancers more susceptible to mutation and the development of resistance to therapy. Due to the critical role of STAT5 in FLT3-ITD AML progression, understanding how this protein is regulated may aid the development of treatment strategies for this disease as well as other STAT5-activated cancers. Poly (ADP-ribose) polymerase 1 (PARP1) is primarily known for its role in DNA repair, but PARP1 can also regulate diverse proteins through its catalytic function of adding poly-ADP-ribosyl groups (PARylating), affecting many other processes. We have previously reported that PARP1 is upregulated in FLT3-ITD AML and contributes to the genomic instability of these cells through upregulation of the highly error-prone DNA repair mechanism, alternative non-homologous end-joining (Alt-NHEJ). In the current study, examination of tyrosine kinase inhibitor (TKI)-resistant FLT3-ITD AML revealed further upregulation of STAT5 and PARP1 protein, indicating highly increased levels of signaling which may represent a therapeutic target. Importantly, examination of the STAT5 protein sequence revealed putative PARylation sites and we now describe a novel binding of PARP1 to STAT5 and direct PARylation of STAT5 in FLT3-ITD AML. Moreover, we demonstrate that PARP1 depletion and inhibition reduces STAT5 protein expression and activity by causing degradation of STAT5 in FLT3-ITD AML cells, suggesting a novel role for PARylation in stabilizing STAT5 protein and potentiating aberrant signaling. Importantly for translational significance, PARP inhibition (PARPi) was cytotoxic in STAT5-activated cancer cells and it was found that PARPi was synergistic with TKI in both TKI-sensitive and -resistant FLT3-ITD AML cells, indicating significant therapeutic potential. Thus, we demonstrate a novel mechanism that may be targeted by PARPi for therapeutic benefit in STAT5-activated leukemias and other cancers.