BACKGROUND: Tumor-associated macrophages (TAMs) in the tumor microenvironment (TME) primarily polarize into the M2-phenotype. Our previous study showed that the small GTPase Rab37 mediates IL-6 trafficking in macrophages for M2 polarization. Here, we uncover an unconventional role of Rab37, independent of vesicle trafficking, in promoting M2 polarization of TAMs. METHODS: The gene profiles in wild-type and Rab37 knockout (KO) bone marrow-derived macrophages (BMDMs) were analyzed using cDNA microarray. The mechanism of Rab37 in regulating the interferon (IFN) pathway was confirmed through in vitro/vivo assays and clinical studies. RESULTS: Type I IFN signaling was highly enriched in BMDMs from Rab37 KO mice. Moreover, Rab37 induction and decreased type I IFN genes were observed in macrophages treated with lung cancer-conditioned medium and epigenetic drugs, indicating an epigenetic regulation of Rab37 in TAMs. Mechanistically, GDP-bound Rab37 interacted with the nuclear localization sequence of STAT1 to sequest it in the cytosol from its transcription activities, thus leading to the downregulation of IFN genes. Clinically, CD163 CONCLUSIONS: Our findings highlight the cytosolic interaction of Rab37-STAT1 in M2 TAM polarization, with CD163