Protein transport. Acts, in coordination with RAB6A, to regulate intra-Golgi retrograde trafficking (By similarity). It is involved in autophagy, acting as a modulator of autophagosome formation. Regulated by a guanine nucleotide-exchange factor (GEF) and a GTPase-activating protein (GAP) and alternates between an inactive GDP-bound and an active GTP-bound form. In vitro, SGSM2 acts as its GAP and inactivates it by stimulating its GTPase activity. Interacts with RIC1 (via C-terminus domain); the interaction is direct with a preference for RAB33B-GTP. Interacts with RGP1 (By similarity). Interacts with ATG16L1; the interaction is important for autophagosome formation (PubMed:18448665). Interacts with ATG16L2; however interaction is approximately hundred times lower than for ATG16L1 (PubMed:22082872). Under starvation conditions punctate RAB33B-positive structures are often observed in the cytoplasm (PubMed:18448665). Ubiquitous. Belongs to the small GTPase superfamily. Rab family.