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We present a mechanochemical, radical-mediated hydroselenation of alkenes under ambient conditions. This method modularly generates alkyl selenyl radicals from elemental selenium and alkyl (pseudo)halides, utilizing PhSiH 3 and H 2 O as hydrogen atom donors, with the water likely originating from the air and/or trace water present in the reagents. ABSTRACT Radical-mediated selenylation has emerged as a powerful strategy for constructing carbon-selenium bonds. While effective for generating aryl selenyl radicals, current approaches for producing alkyl selenyl radicals remain underdeveloped. In this study, we present a mechanochemical approach that enables the modular generation of alkyl selenyl radicals from elemental selenium and alkyl halides, thereby advancing the hydroselenation of alkenes. The modular generation of alkyl selenyl radicals utilizes abundant and commercially available alkyl (pseudo)halides as radical precursors, activated through mechanical force to overcome selenium's inherent inertness. The reaction proceeds under ambient conditions with minimal solvent usage, achieving a broad substrate scope encompassing alkyl iodides, bromides, chlorides, and sulfonates. Notably, mechanistic experiments and density functional theory (DFT) calculations indicate that both PhSiH 3 or/and H 2 O can function as hydrogen atom donors through a radical pathway. This finding appears to be significant in the case of water, where the oxophilicity of silicon likely provides the driving force, pointing to a plausible silicon-mediated activation of water. We therefore propose a mechanochemical silicon-mediated radical process where coordination-induced bond weakening could enable intermolecular hydrogen atom transfer (HAT) from H 2 O to carbon radicals.