In carbohydrate chemistry, anomers (from Greek ἄνω 'up, above' and μέρος 'part') are specific types of stereoisomers found in sugars.

Many common sugars, such as glucose, exist in both a linear (or open-chain) form and a cyclic (or ring) form. The ring is formed when one end of the sugar molecule connects to the other end. The carbon atom where this ring closure occurs is called the anomeric carbon. Depending on the direction from which the connection is made, this anomeric carbon can have its new group (–OH) pointing in one of two distinct orientations, typically visualized as "up" or "down" in a standard diagram. These two resulting molecules are the anomers and are labeled with the Greek letters alpha (α) or beta (β).

More formally, an anomer is an epimer at the hemiacetal/hemiketal carbon atom in a cyclic saccharide. The process of one anomer converting to the other is known as anomerization. Because they have different three-dimensional structures, anomers have distinct physical properties, such as melting point and specific rotation.

Every two anomers are designated alpha (α) or beta (β), according to the configurational relationship between the anomeric centre and the anomeric reference atom, hence they are relative stereodescriptors. The anomeric centre in hemiacetals is the anomeric carbon C-1; in hemiketals, it is the carbon derived from the carbonyl of the ketone (e.g. C-2 in D-fructose). In aldohexoses the anomeric reference atom is the stereocenter that is farthest from the anomeric carbon in the ring (the configurational atom, defining the sugar as D or L). For example, in α-D-glucopyranose the reference atom is C-5.

If in the cyclic Fischer projection the exocyclic oxygen atom at the anomeric centre is cis (on the same side) to the exocyclic oxygen attached to the anomeric reference atom (in the OH group) the anomer is α. If the two oxygens are trans (on different sides) the anomer is β.

Anomerization is the process of conversion of one anomer to the other. For reducing sugars, anomerization is referred to as mutarotation and occurs readily in solution and is catalyzed by acid and base. This reversible process typically leads to an anomeric mixture in which eventually an equilibrium is reached between the two single anomers.

The ratio of the two anomers is specific for a given sugar solution. For example, regardless of the configuration of the starting D-glucose, a solution will gradually move towards being a mixture of approximately 64% β-D-glucopyranoside and 36% of α-D-glucopyranose. As the ratio changes, the optical rotation of the mixture changes; this phenomenon is called mutarotation. While enantiomers have equal and opposite specific rotations, anomers (which are diastereomers) do not follow this rule and can have different specific rotations both in magnitude and sign.

The specific rotation ${\displaystyle [\alpha ]_{\lambda }^{T}}$ is a physical property defined as the optical rotation ${\displaystyle \alpha }$ at a path length ${\displaystyle l}$ of 1 dm, a mass concentration ${\displaystyle c}$ (or density ${\displaystyle \rho }$ in pure compounds) in g/cm³, a temperature ${\displaystyle T}$ (usually 20 °C) and a light wavelength ${\displaystyle \lambda }$ (usually sodium D line at 589.3 nm):