Oedogonium is a genus of filamentous, free-living green algae. It was first described in 1820 by Johann Heinrich Friedrich Link, and later given its name by German scientist K. E. Hirn.

The morphology of Oedogonium is unique, with an interior and exterior that function differently from one another and change throughout its life cycle. These algae reside in freshwater ecosystems in both hemispheres and are both benthic and planktonic in nature. They form algal patches on the water's surface and so interact closely with a multitude of other algae. These filamentous cells' life cycles include both sexual and asexual reproduction, depending on the life cycle stage.

Although quite common, Oedogonium is difficult to identify to species since key definitive markers are only present during reproduction, which is an uncommon life stage among this genus. Oedogonium has been found to be important in the fixation of heavy metals in freshwater ecosystems.

First named Oedogoniaceen (in German), the name Oedogoniales is derived from the Latin oedos (meaning swelling or tumor) and gonos (meaning offspring or seed). This name describes the morphology that Hirn witnessed during Oedogonium sexual and asexual reproduction and later described in his publication, “Monographie und iconographie der Oedogoniaceen."

Oedogonium species were first reported in the late 19th century by Hilse (1860), Gołowin (1964), Kirchner (1878), Kozłowski (1895) and Gutwiński (1897). Mrozińska was the first to examine the genus in terms of morphology, ecology and distribution and described more than 400 species, mainly from southern Poland.

In 1900, Hirn wrote a monograph concerning his finding of a new taxon, to which he gave the name Oedogoniaceen. Hirn's monograph now serves as the starting point of nomenclature for the family Oedogoniaceae, an exception to Linnaeus' Species plantarum as a starting point for other algae.

In 1991, a paper by Mrozińska presented a new taxonomic classification of the genus Oedogonium, and a proposed division into two sections: I. Monospermatozoideae and II. Dispermatozoideae. These sections were based on the different number of spermatozoids the antheridial (male sex organ containing) cell expresses. This classification is not widely accepted.^{[citation needed]}

Cells of the genus Oedogonium are narrow and cylindrical in shape. The algal body consists of green, un-branched, multi-cellular filaments, arranged end to end. Every cell of the filamentous algal body (called the thallus) is similar in shape apart from the apical cell (the uppermost) and the holdfast cell (the lowermost). The apical cell is wider and rounded at its tip relative to the other cells of the thallus. The holdfast cell produces elongated growths from both unattached sides which aid in firmly attaching the filament to a substrate. The holdfast is also the only colorless cell of the filament.