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The regulation of endosome dynamics is crucial for fundamental cellular functions, such as nutrient intake/digestion, membrane protein cycling, cell migration and intracellular signalling. Here, we show that a novel lipid raft adaptor protein, p18, is involved in controlling endosome dynamics by anchoring the MEK1–ERK pathway to late endosomes. p18 is anchored to lipid rafts of late endosomes through its N‐terminal unique region. p18^−/− mice are embryonic lethal and have severe defects in endosome/lysosome organization and membrane protein transport in the visceral endoderm. p18^−/− cells exhibit apparent defects in endosome dynamics through perinuclear compartment, such as aberrant distribution and/or processing of lysosomes and impaired cycling of Rab11‐positive recycling endosomes. p18 specifically binds to the p14–MP1 complex, a scaffold for MEK1. Loss of p18 function excludes the p14–MP1 …