A stout with a proportion of oats, normally a maximum of 30%, added during the brewing process. Oatmeal stouts do not usually taste specifically of oats, but have a smoothness from the high content of proteins, lipids (includes fats and waxes) and gums imparted by the use of oats.

Even though a larger proportion of oats in beer can lead to a bitter or astringent taste, oats were a common ingredient in ale during the medieval period in Europe and proportions up to 35% were standard. Despite some areas of Europe, such as Norway, still clinging to the use of oats in brewing until the early part of the 20th century, the practice had largely died out by the 16th century.

There was a revival of interest in using oats during the end of the 19th century, when (supposedly) restorative, nourishing and invalid beers were popular.

In the 20th century, many oatmeal stouts contained only a minimal amount of oats and they are now rare.