Imperial stout, also known as "Russian Imperial stout", is a strong dark beer in the style that was brewed in the 18th century by Thrale's Anchor Brewery in London for export to the court of Catherine II of Russia. In 1781 the brewery changed hands and the beer became known as "Barclay Perkins Imperial Brown Stout". It was shipped to Russia by Albert von Le Coq who was awarded a Russian royal warrant which entitled him to use the name "Imperial". A recipe from 1856 shows it had an original gravity of 1.107 (almost certainly over 10% abv) and over 10 pounds of hops to the barrel. When Barclay's brewery was taken over by Courage in 1955, the beer was renamed "Courage Imperial Russian Stout" and it was brewed sporadically until 1993.

In Canada, Imperial Stout was produced in Prince Albert first by Fritz Sick, and then by Molson following a 1958 takeover. Denmark's Wiibroe Brewery launched its 8.2 percent Imperial Stout in 1930. The first brewery to brew an Imperial Stout in the United States was Bert Grant's Yakima Brewing.

Imperial stouts have a high alcohol content, usually over 9% abv, and are among the darkest available beer styles. Samuel Smith's brewed a version for export to the United States in the early 1980s, and today imperial stout is among the most popular beer styles with U.S. craft brewers. American interpretations of the style often include ingredients such as vanilla beans, chili powder, maple syrup, coffee, and marshmallows. Many are aged in bourbon barrels to add additional layers of flavour. The word "Imperial" is now commonly added to other beer styles to denote a stronger version, hence Imperial IPAs, Imperial pilsners etc.