The synthesis of a new functional group in the form of selenyl-substituted aziridines is described. Selenoaziridines are stereoselectively prepared by functionalization of intact aziridine precursors involving radical and anionic intermediates. Radicals are generated from cis-N-Ts iodoaziridines by activation of the C–I bond using alkoxides as a source of single electrons. These form predominantly trans-substituted seleno-aziridines dependent on the size of the diselenide. cis-Aziridinyllithiums generated by Li–I exchange also react with diselenides stereospecifically to form a range of cis-selenoaziridines. Proposals for the stereochemical outcome are presented.