Müllerian mimicry is a natural phenomenon when two or more poisonous species, that may or may not be closely related and share one or more common predators, have come to mimic each other's warning signals. It is named after the German naturalist Fritz Müller, who first proposed the concept in 1878.

The phenomenon can be understood by imagining two poisonous species that do not resemble one another and are also prey to a common predator. Occasionally, individuals of the predatory third species will encounter one or the other type of noxious prey, and thereafter avoid it. Predators that avoid only one or the other type of harmful species provide no benefit to the species that is not avoided. Therefore, there is an evolutionary advantage to be gained in the gradual approach in appearance of the two prey species. This is because a predator that learns to avoid either species in a pair of species exhibiting Müllerian mimicry learns, in effect, to avoid both.

This strategy is typically contrasted with Batesian mimicry, in which one harmless species adopts the appearance of another, harmful species to gain the advantage of predators' avoidance. However, because comimics may have differing degrees of protection, the distinction between Müllerian and Batesian mimicry is not absolute, and there can be said to be a spectrum between the two forms. Additionally, a species may be a Batesian mimic to one predator and a Müllerian mimic to another. While Batesian and Müllerian mimicry are commonly given examples of mimicry, there is often little or no mention of other forms. There are many other types of mimicry however, some very similar in principle, others far separated. For example in aggressive mimicry a predator might mimic the food of its prey, luring them towards it and improving its foraging success.