Pilsener or pilsner is a pale lager, developed in the 19th century city of Pilsen in Bohemia (today Plzen in the Czech Republic).

Until the 1840s, most Bohemian beers were top-fermented, dark and cloudy. The taste and standards of quality often varied to the worse, and in 1838, consumers even dumped whole barrels to show their dissatisfaction. The citizens of Pilsen decided in 1839 to found and build a brewery of their own, the Bürger-Brauerei (Burgess' Brewery, Mestansky pivovar), which should brew beer according to the Bayerischen Brauart, the Bavarian style of brewing. Bavarian brewers had begun experiments with the storage of beer in cool caves using bottom-fermenting yeasts, which improved the beer's clarity, flavour, and shelf-life.

The Bürger-Brauerei using new techniques and the newly available paler malts, presented the first batch of modern pilsener on October 5, 1842. The combination of pale colour from the new malts, Pilsen's remarkably soft water, noble hops from nearby Saaz (pronounced "Zaats") and Bavarian-style lagering produced a clear, golden beer which was regarded as a sensation.

A modern pilsener has a very light, clear color from pale to golden yellow, and a distinct hop aroma and flavor. Czech pilseners tend toward a lighter flavor while those in a German style can be more bitter or even "earthy" in flavor.

While pilsener is best defined in terms of its characteristics and heritage, the term is also used by some brewers (particularly in North America) to indicate their "premium" beer, whether or not it has a particular hop character. It is generally regarded as being different from other pale lagers by a more prominent hop character, particularly from the use of Saaz noble hops.