Kiritimati (also known as Christmas Island) is a Pacific Ocean atoll in the northern Line Islands. It is part of the Republic of Kiribati. The name is derived from the English word "Christmas" written in Gilbertese according to its phonology, in which the combination ti is pronounced s, giving [kiˈrɪsmæs].

The island has the greatest land area of any atoll in the world, about 388 square kilometres (150 square miles); its lagoon is roughly the same size. The atoll is about 150 km (93 mi) in perimeter, while the lagoon shoreline extends for over 48 km (30 mi). Kiritimati comprises over 70% of the total land area of Kiribati, a country encompassing 33 Pacific atolls and islands.It lies 232 km (144 mi) north of the equator, 2,160 km (1,340 mi) south of Honolulu, and 5,360 km (3,330 mi) from San Francisco. Kiritimati is in the world's farthest forward time zone, UTC+14, and is therefore one of the first inhabited places on Earth to experience New Year's Day. (see also Caroline Atoll, Kiribati). Although it lies 2,460 km (1,530 mi) east of the 180th meridian, the Republic of Kiribati realigned the International Date Line in 1995, placing Kiritimati to the west of the dateline.

Nuclear tests were conducted on and around Kiritimati by the United Kingdom in the late 1950s, and by the United States in 1962. During these tests, the island was not evacuated, exposing the i-Kiribati residents and the British, New Zealand, and Fijian servicemen to nuclear radiation.

The entire island is a Wildlife Sanctuary; access to five particularly sensitive areas is restricted.