Vitamin A<sub>2</sub> is a subcategory of vitamin A.
As with all vitamin A forms, A<sub>2</sub> can exist as an aldehyde, Dehydroretinal (3,4-dehydroretinal), an alcohol, 3,4-dehydroretinol (vitamin A<sub>2</sub> alcohol) or an acid, 3,4-dehydroretinoic acid (vitamin A<sub>2</sub> acid). Many cold-blooded vertebrates use the aldehyde for their visual system to obtain a red-shifted sensitive spectrum.
Human skin naturally contains the alcohol form. In humans, CYP27C1 converts ordinary A<sub>1</sub> (all-trans retinoids) to A<sub>2</sub>. The enzyme also converts 11-cis-retinal.
Vitamin A<sub>2</sub> was first identified by Richard Alan Morton using newly-developed absorption spectroscopy in 1941.