Diiodomethane or methylene iodide, commonly abbreviated "MI", is an organoiodine compound.

Diiodomethane is a very dense colorless liquid; however, it decomposes upon exposure to light liberating iodine, which colours samples brownish.

It can also be used as an optical contact liquid, in conjunction with the gemmological refractometer, for determining the refractive index of certain gemstones.

[4] In fact the Simmons–Smith reaction does not produce free carbene but proceeds via Zn-CH2I intermediates.

The synthesis of Fe2(CH2)(CO)8 illustrates this reactivity:[5] Diiodomethane can be prepared from the widely available solvent dichloromethane by the action of sodium iodide in acetone in the Finkelstein reaction:[6] It can also be prepared by reducing iodoform with elemental phosphorus[7] or sodium arsenite:[6] Alkyl iodides are alkylating agents, which are potential mutagens.