The monarch (Danaus plexippus) is a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all butterflies. Since the 19th century, it is also found in New Zealand, and has been known in Australia since 1871. In Australia it is also known as the wanderer butterfly. In Europe it is resident in the Canary Islands and Madeira, and is found as a migrant in Russia, Azores, Sweden and Spain. Its wings feature an easily recognizable orange and black pattern. The viceroy butterfly has a similar size, color, and pattern, but can be distinguished by an extra black stripe across the hindwing. Female monarchs have darker veins on their wings, and the males have a spot in the center of each hindwing from which pheromones are released.

The monarch’s wingspan ranges from 8.9–10.2 centimetres. The upper side of the wings is tawny-orange, the veins and margins are black, and in the margins are two series of small white spots. The fore wings also have a few orange spots near the tip. The underside is similar but the tip of the forewing and hindwing are yellow-brown instead of tawny-orange and the white spots are larger. The male has a black patch of sex-scales on the hindwings, and the black veins on its wing are narrower than the female’s. The male is also slightly larger.

Monarchs are especially noted for their lengthy annual migration. In North America they make massive southward migrations starting in August until the first frost. A northward migration takes place in the spring. In Australia, they make limited migrations in cooler areas, but the blue tiger butterfly is better known in Australia for its lengthy migration. Female monarchs deposit eggs for the next generation during these migrations. By the end of October, the population of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in the Mexican states of Michoacán and México. The western population overwinters in various sites in central coastal and southern California, United States, notably in Pacific Grove and Santa Cruz.

Monarch butterflies are one of the few insects capable of making transatlantic crossings. They are becoming more common in Bermuda due to increased usage of milkweed as an ornamental plant in flower gardens. Monarch butterflies born in Bermuda remain year round due to the island's mild climate. A few monarchs turn up in the far southwest of Great Britain in years when the wind conditions are right, and have been sighted as far east as Long Bennington. Monarchs can also be found in New Zealand. On the islands of Hawaii no migrations have been noted.