Oligosynthetic (from the Greek ολίγοι, meaning "few") is a hypothetical designation for a language using an extremely small array of morphemes, perhaps numbering only in the hundreds, which combine synthetically to form statements. The chief difference between a polysynthetic and an oligosynthetic language is the total number of morphemes, which for the latter would be much smaller. Such a language would possess, in a manner of speaking, an "oligarchy" of morphemes. Speech would depend heavily on the creation of lengthy compound words, to an extent far exceeding that of regular synthetic languages.

At this time, oligosynthesis is almost entirely theoretical, as linguists have yet to discover an actual human language that meets the criteria for classification. Certain Native American languages, namely Nahuatl and Blackfoot, have in the past been claimed to exhibit oligosynthetic qualities (most notably by Benjamin Whorf). However, the linguistic community has largely rejected these assertions, preferring to categorize Nahuatl and Blackfoot as polysynthetic.

Indeed, the very concept of oligosynthetic language has always been regarded as somewhat far-fetched. The fact that no existing language, living or dead, has been demonstrably shown to exhibit oligosynthetic properties has led some linguists to regard true oligosynthesis as impossible (or at any rate, wildly impractical) for productive use by human beings.