Lager Yeast, Saccharomyces pastorianus, is a bottom fermenting yeast used for brewing lager style beers. It is physiologically distinct from the top fermenting (so called because it forms a thick foam at the top of the wort during fermentation) ale yeast S. cerevisiae in its abilities to ferment at cooler temperatures and to ferment the sugar melibiose. Lager yeast also typically ferments more of the sugars than ale yeast, leading to a crisper taste.

At the genetic level, lager yeast is a hybrid organism between two closely related yeast species, S. cerevisiae and S. bayanus. It is thought to have arisen in response to selective pressures from cold brewing temperatures, and, notably, S. bayanus is much more cold tolerant that S. cerevisiae. Thus, the presence of these two genomes in the hybrid S. pastorianus may have led to its observed ability to carry out fermentation better at cold temperatures than S. cerevisiae alone. This selection for a cooler fermenting yeast may have taken place during successive rounds of cold-temperature fermentations resulting from a 16th-century Bavarian law that prohibited brewing during summer months due to the inferior quality of summer-brewed beers. Many isolates of S. pastorianus have been collected from breweries since Hansen pioneered pure culturing of yeast in the late 1800s, and recent work has shown that all lager strains in use today likely descended from one of two hybridization events between S. cerevisiae and S. bayanus. In both cases, it is probable that the S. cerevisiae parent of these two original S. pastorianus strains was a strain already being used for brewing ale.

See also bottom fermentation and yeast.