Nauru is a small, oval-shaped island in the southwestern Pacific Ocean, 42 km (26 miles) south of the Equator. The island is surrounded by a coral reef, which is exposed at low tide and dotted with pinnacles. The reef is bound seaward by deep water, and on the inside by a sandy beach. The presence of the reef has prevented the establishment of a seaport, although 16 artificial channels have been made in the reef to allow small boats to access the island. A 150 to 300 meter (about 500 to 1000 ft.) wide fertile coastal strip lies inland from the beach. Coral cliffs surround Nauru's central plateau, which is known as "Topside". The highest point of the plateau, called the Command Ridge, is 71 meters above sea level. The only fertile areas on Nauru are the narrow coastal belt, where coconut palms flourish. The land surrounding Buada Lagoon supports bananas, pineapples, vegetables, pandanus trees, and indigenous hardwoods such as the tomano tree. The population of Nauru is concentrated in the coastal belt and around Buada Lagoon.

Nauru was one of three great phosphate rock islands in the Pacific Ocean (the others were Banaba (Ocean Island) in Kiribati and Makatea in French Polynesia). However, the phosphate reserves on Nauru are depleted for all practical purposes. Phosphate mining in the central plateau has left a barren terrain of jagged limestone pinnacles up to 15 meters (49 ft.) high. A century of mining has stripped and devastated about 80% of the land area. Mining has also impacted the surrounding Exclusive Economic Zone, with 40% of marine life estimated to have been killed by silt and phosphate runoff.

There are quite limited natural fresh water resources on Nauru. Rooftop storage tanks collect rainwater, but the islanders are mostly dependent on a single, aging desalination plant. Nauru's climate is hot and very humid year-round -- because of the proximity of the land to the Equator and the ocean. Nauru is hit by monsoon rains between November and February. Annual rainfall is highly variable and is influenced by the El Niño-Southern Oscillation, with several significant recorded droughts. The temperature on Nauru ranges between 26 and 35° Celsius (79 to 95° Fahrenheit) during the day and between 25 and 28° Celsius (77 to 82° F.) at night. As an island nation, Nauru is quite vulnerable to climate change and sea level change, but to what degree is difficult to predict. At least 80% of the land of Nauru is well-elevated, but this area will be uninhabitable until the phosphate mining rehabilitation program is implemented. Also, the agricultural area of Nauru is quite close to the seashore.

There are only about 60 recorded vascular plant species native to the island, none of which are endemic. Coconut farming, mining, and introduced species have caused serious disturbance to the native vegetation. There are no native land mammals, but there are native birds, including the endemic Nauru Reed Warbler, insects, and land crabs. The Polynesian rat, cats, dogs, pigs, and chickens have been introduced to Nauru from ships, either accidentally or on purpose.