A vitamer (/ˈvaɪtəmər/) is any form in which some vitamin occurs. Each vitamer of a particular vitamin is a compound that performs the functions of that vitamin and prevents the symptoms of deficiency of the vitamin.

Early research identified vitamins by their ability to cure vitamin-specific deficiency diseases. For example, vitamin B₁ was first identified as a substance that prevented and treated beriberi. Subsequent nutrition research has revealed that all vitamers exhibit biological activity against their specific vitamin deficiency, although different vitamers exhibit different potencies against those diseases.

A set of vitamers with related biological activity are grouped together by a general name, or generic descriptor, that refers to similar compounds with the same vitamin function. For example, vitamin A is the generic descriptor for the class of vitamin A vitamers which include retinol, retinal, retinoic acid, and provitamin carotenoids such as beta-carotene among others.

Vitamers often have subtly different properties from their primary, or most common form. These differences include abundance in the typical diet, bioavailability, toxicity, physiological activities, and metabolism. Some vitamers are associated with different benefits for health compared to other forms of the same vitamin.

Folic acid, a vitamer of vitamin B₉ commonly added to fortified foods and dietary supplements, is 0.7–1.0 times more bioavailable than vitamers of vitamin B₉ found in minimally processed foods. Differences in digestion and absorption account for the notable differences in bioavailability between vitamers of vitamin B₉. Forms of vitamin B₉ that occur in minimally processed foods, sometimes referred to as "food folates", require digestion by enzymatic hydrolysis prior to absorption whereas folic acid does not.  

Some vitamins have toxic effects when consumed in excess amounts and certain vitamers have a greater potential for toxicity compared to other forms of the same vitamin. For example, hypervitaminosis A is a toxicity syndrome caused by excess consumption of retinoid vitamers of vitamin A such as retinol, retinal, and retinoic acid. In contrast, provitamin A carotenoids such as beta-carotene are not associated with these toxic effects.

Nicotinic acid and nicotinamide are two vitamers of vitamin B₃ that exhibit differences in metabolism. Large, pharmaceutical doses of the nicotinic acid are used under medical supervision as a treatment for hypercholesterolemia. High doses of nicotinic acid are also associated with a potential for adverse effects, most commonly a niacin flush reaction that is characterized by redness or flushing of the skin, sensations of heat, itching, and tingling. The nicotinamide vitamer of vitamin B₃ does not exhibit the same therapeutic effect for treatment of hypercholesterolemia, but also does not cause a niacin flush reaction and is not associated with the same adverse effects as nicotinic acid.

As part of an overall diet, minimally processed foods provide a number of different naturally occurring vitamers. This is frequently in contrast to fortified foods and dietary supplements which generally provide vitamins as a single vitamer. Vitamin E, vitamin B₆, and vitamin B₉ are three examples.