The Mussau monarch (Symposiachrus menckei), also known as the white-breasted monarch, is a species of bird in the family Monarchidae.

It is endemic to the Bismarck Archipelago of Papua New Guinea.

Its natural habitats are subtropical or tropical moist lowland forests and rural gardens.

The Mussau monarch was originally placed in the genus Monarcha until moved to Symposiachrus in 2009.

The species name is after Bruno Mencke who supported the expedition on which the species was collected by Oskar Heinroth.