The Papuan Harrier (Circus spilonotus spilothorax), currently classified as a subspecies of the Eastern Marsh-Harrier (C. spilonotus), is endemic to the island of New Guinea and may be in need of conservation attention because of threats from grassland burning. I here detail the discovery of the first known nests in lowland Papua New Guinea and provide egg dimensions and prey data. Both nests were initiated in early April, in damp rank grassland, and contained three small chicks in mid-May. The only egg measurements, combined with one previously published record, suggest large egg volume and concomitant large female body size (estimated to be ca. 890 g). At this size, this may be the world's largest harrier. Fire destroyed both nests within 5 wk of their discovery. An atypically slow foraging style and a preponderance of game birds and large rats (Rattus spp.) in the pellets and prey remains are consistent with large body size. Further studies of the bird's ecology and breeding are needed for a comprehensive understanding of its conservation status and threats to its population.