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Purpose: Targeting of KIT and platelet-derived growth factor receptor (PDGFR) tyrosine kinases by imatinib is an effective anticancer strategy. However, mutations of the gatekeeper residue (T670 in KIT and T681 in PDGFRβ) render the two kinases resistant to imatinib. The aim of this study was to evaluate whether sorafenib (BAY 43-9006), a multitargeted ATP-competitive inhibitor of KIT and PDGFR, was active against imatinib-resistant KIT and PDGFRβ kinases. Experimental Design: We used in vitro kinase assays and immunoblot with phosphospecific antibodies to determine the activity of sorafenib on KIT and PDGFRβ kinases. We also exploited reporter luciferase assays to measure the effects of sorafenib on KIT and PDGFRβ downstream signaling events. The activity of sorafenib on interleukin-3–independent proliferation of Ba/F3 cells expressing oncogenic KIT or its imatinib-resistant T670I mutant was also tested. Results: Sorafenib efficiently inhibited gatekeeper mutants of KIT and PDGFRβ (IC50 for KIT T670I, 60 nmol/L; IC50 for PDGFRβ T681I, 110 nmol/L). Instead, it was less active against activation loop mutants of the two receptors (IC50 for KIT D816V, 3.8 μmol/L; IC50 for PDGFRβ D850V, 1.17 μmol/L) that are also imatinib-resistant. Sorafenib blocked receptor autophosphorylation and signaling of KIT and PDGFRβ gatekeeper mutants in intact cells as well as activation of AP1-responsive and cyclin D1 gene promoters, respectively. Finally, the compound inhibited KIT-dependent proliferation of Ba/F3 cells expressing the oncogenic KIT mutant carrying the T670I mutation. Conclusions: Sorafenib might be a promising anticancer agent for patients carrying KIT and PDGFRβ gatekeeper mutations.