A lek (from Swedish lek, a noun which typically denotes pleasurable and less rule-bound games and activities) is a gathering of the males of certain species of animals for the purposes of competitive mating display.

The places where leks occur are often the same from one mating season to the next, and are known as lek sites. The term was originally used most commonly for Black Grouse (orrlek) and for Capercaillie (tjäderlek), and lekking behaviour is quite common in birds of this type, such as Sage Grouse. However it is also shown by birds of other families, such as the Ruff and Musk Duck, by some mammals such as the Uganda kob (a grazing antelope) and by some species of fish. The rut of deer is also very similar. There is some dispute among ethologists as to whether the lekking behaviour shown by animals of widely different groups should really be treated as the same, and in particular whether similar selective pressures have led to their emergence.