There are 15 major islands, spread over 2.2 million km² of ocean, divided into two distinct groups: the Southern Cook Islands, and the Northern Cook Islands of coral atolls.

The islands were formed by volcanic activity; the northern group is older and consists of six atolls (sunken volcanoes topped by coral growth). The climate is moderate to tropical.

The 15 islands and two reefs are grouped as follows:

Southern Cook Islands:
Nga-pu-Toru ("Three Roots", the group name for the easternmost islands of Southern Group):
Atiu (Enua Manu or Island of Birds)
Ma'uke (Akatokamanava)
Mitiaro (Nukuroa)
Takutea
Remaining islands of the South:
Aitutaki (Araura Enua)
Mangaia (Auau Enua)
Rarotonga (Tumutevarovaro) and also with the capital, Avarua.
Palmerston Island (Pamati) sometimes grouped with the Northern Group.
Manuae
Winslow Reef (submerged)
Northern Cook Islands:
Manihiki (Te Fuinga o Niva)
Nassau (Te Nuku o Ngalewu )
Penrhyn Island (Tongareva or Mangarongaro)
Pukapuka (Te Ulu o te Watu)
Rakahanga (Tapuahua)
Suwarrow also called Suvorov
Tema Reef (submerged)