A monosaccharide (sugar) in which the carbonyl group (or potential carbonyl group when in cyclic form) is not terminal, ie. it is ketonic. The trivial names of many ketoses end in "+ulose", as do the systematic names (such as D-erythro-ribulose).

The ketoses can be categorised by the number of carbon atoms they contain. The number of possible sugars increases with the number of carbon atoms present, as the number of chiral centers increases. For instance, there is only one possible ketotriose, as there is no chiral center in the molecule. There are two possible ketotrioses due to the presence of a chiral center - a carbon atom which has a "handedness" with regards to the atoms it is bonded to such that it is not identical with it's mirror image, in much the same way as left and right shoes are nonidentical. This gives rise to two possible sugars that are mirror-images of each other, that is to say an enantiomeric pair. These are designated D- and L-glycero-tetrulose. For the ketopentoses, there are two chiral centers, giving rise to 22 possible combinations, or two pairs of enantiomers. Similarly, ketohexoses have 23 possible combinations, falling into four pairs.

Ketotrioses 3 carbon atoms, 1 possible.
glycerone (dihydroxyacetone)

Ketotetroses 4 carbon atoms, 1 enantiomeric pair possible.

Ketopentoses 5 carbon atoms, 2 enantiomeric pairs possible.
D/L-ribulose, D/L-xylulose

Ketohexoses 6 carbon atoms, 4 enantiomeric pairs possible.
D/L-fructose, D/L-psicose, D/L-sorbose, D/L-tagatose