Rio Muni (called Mbini in Fang), mainland geographical region of Equatorial Guinea, covering 26,000 km².

Rio Muni was ceded by Portugal to Spain in 1778. The Spanish had hoped to collect slaves to work in their other overseas possessions, but their settlers died of yellow fever and the area was deserted. Cocoa and timber became major industries upon recolonization. Rio Muni became a province of Spanish Guinea along with Bioko in 1959.

Rio Muni currently has a population of about 300,000, mainly composed of ethnic Fang tribes. It is split into four political jurisdictions: Centro Sur, Kie-Ntem, Litoral and Wele-Nzas. The largest city is Bata. Other major towns include Evinayong, Ebebiyin, Acalayong, Acurenam, Mongomo, Sevilla de Niefang, Valladolid de los Bimbiles and Mbini.

Postage stamps

Residents of Rio Muni originally used the postage stamps of Spanish Guinea, until 1960 when the Spanish government decreed the use of separate issues for Rio Muni and Fernando Po (Bioko). The first definitive series appeared in that year, consisting of nine values, 25 c to 10 p, all with the same design showing a missionary and a native boy reading, and inscribed "RIO MUNI".

Issues from 1961 on added the inscription "ESPAÑA"; typically 2-3 issues per year, consisting of 2-4 stamps each, and usually depicting local plants and animals. Another definitive series appeared in 1964, also with nine values. The last issue of Rio Muni was a set of three signs of the Zodiac issued on 25 April 1968; Equatorial Guinea became independent in October, and issued its own stamps subsequently.

All of the approximately 85 stamps of Rio Muni are readily available to collectors, almost all at the minimum price.