The title compound, C(6)H(13)O(3)PSe, was obtained in the reaction of 5,5-dimethyl-2-oxo-2-seleno-1,3,2-dioxaphospho-r-inane potassium salt with methyl iodide. The seleno-methyl group is in the axial position in relation to the six-membered dioxaphospho-rinane ring.