The Grb2 adaptor protein is best known for its role in signaling to the small GTPase p21(ras), mediated through its interaction with the SOS guanine nucleotide exchange factor. Here, we demonstrate that Grb2 also signals to Rab5, a small GTPase that plays a key role in early endocytic trafficking. Grb2 functions through association with RN-tre, a GTPase-activating protein for Rab5. Grb2 and RN-tre associate both in vitro and in vivo, with interaction mediated by both SH3 domains of Grb2 and extended proline-rich sequences in RN-tre. Association between Grb2 and RN-tre is constitutive and occurs independently of Eps8, a previously identified binding partner of RN-tre. Epidermal growth factor (EGF) stimulates recruitment of RN-tre to the EGF receptor (EGFR) in a Grb2-dependent manner. Grb2 and the EGFR are internalized and co-localized in endocytic vesicles in response to EGF. Overexpression of RN-tre blocks the internalization of both proteins, consistent with its function as a negative regulator of Rab5 and endocytosis. Strikingly, RN-tre does not block EGF-induced internalization of a Grb2 mutant deficient in RN-tre binding. These results 1) suggest that the ability of RN-tre to inhibit internalization of the EGFR requires Grb2-mediated binding to the receptor and 2) identify Grb2 as a critical regulator of Rab5 and EGFR endocytosis.