Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound with chemical formula CH2(OH)2. It is the simplest geminal diol, and formally the second simplest carbohydrate (after formaldehyde).

Methanediol is a product of the hydration of formaldehyde H2C=O, and predominates in water solution: the equilibrium constant being about 103, and a 5% by weight solution of formaldehyde in water is 80% methanediol.

The compound is of some relevance to astrochemistry.

Methanediol is listed as one of the main ingredients of "Brazilian blowout", a hair-straightening formula marketed in the US. The equilibrium with formaldehyde may cause some concern as formaldehyde in hair straighteners is a health hazard.