Protein phosphatase 2A (PP2A) is a critical tumor suppressor frequently inactivated in hematological malignancies, yet the mechanisms underlying this dysregulation remain poorly defined. Here, we investigated a novel interaction between the PP2A catalytic subunit (PP2Ac) and the adaptor protein CIN85. Analyzing transcriptomic datasets from AML, CML, CLL, and ALL patients, alongside functional validation in K562 and THP1 leukemia cell lines via RT-qPCR, immunoblotting, phosphatase activity assays, pharmacological modulation (FTY720 and Okadaic acid), siRNA-mediated knockdown, and proliferation assays, we found that CML and CLL exhibit reduced PP2A and elevated CIN85, with activated AKT, SRC, and GSK-3β levels. Pharmacological activation of PP2A reduced AKT and GSK-3β phosphorylation, while inhibition had opposing effects, confirming PP2A’s regulatory role. Crucially, CIN85 silencing markedly enhanced PP2A phosphatase activity without changing PP2Ac protein levels, leading to robust dephosphorylation of AKT, SRC, and GSK-3β and significantly suppressing leukemic cell proliferation. Collectively, these results identify CIN85 as a previously unrecognized endogenous negative regulator that directly suppresses PP2A function in leukemia. Importantly, disrupting the PP2Ac-CIN85 complex effectively restores PP2A tumor-suppressive activity, dampens oncogenic signaling, and inhibits growth, providing mechanistic clarity and positioning this protein-protein interface as one of the possible therapeutic targets for treating hematological cancers.