The country's geography is diverse and, in places, extremely rugged. A spine of mountains, the New Guinea Highlands, runs the length of the island of New Guinea, forming a populous highlands region mostly covered with tropical rainforest. Dense rainforests can be found in the lowland and coastal areas as well as very large wetland areas surrounding the Sepik and Fly rivers. This terrain has made it difficult for the country to develop transportation infrastructure. In some areas, airplanes are the only mode of transport. The highest peak is Mount Wilhelm at 4,509 metres (14,793 ft). Papua New Guinea is surrounded by coral reefs which are under close watch to preserve them.

The country is situated on the Pacific Ring of Fire, at the point of collision of several tectonic plates. There are a number of active volcanoes, and eruptions are frequent. Earthquakes are relatively common, sometimes accompanied by tsunamis.

The mainland of the country is the eastern half of New Guinea island, where the largest towns are also located, including the capital Port Moresby and Lae; other major islands within Papua New Guinea include New Ireland, New Britain, Manus and Bougainville.

Papua New Guinea is one of the few regions close to the equator that experience snowfall, which occurs in the most elevated parts of the mainland.