We present a compilation and analysis of the vascular plants, vegetation, and ethnobotany of Banaba (Ocean) Island in the Republic of Kiribati in the tropical Central Pacific Ocean. It is based on a 2005 field survey, plus analysis of available information in the literature. The flora of the small, raised phosphatic limestone island of Banaba is very limited compared to the floras of larger limestone and volcanic islands of the tropical Pacific Ocean. Isolation, small size, prolonged droughts and water scarcity, almost 80 years of open-cast phosphate mining, abandonment of the island by most people after cessation of mining in 1979, and widespread destruction, relocation and death of the Banaban people during and after World War II have led to serious degradation, disturbance, displacement and loss of the flora, vegetation, knowledge and ancient cultural traditions related to plants. This paper attempts for the first time to document the nature of Banaba’s flora and vegetation and provide background on the reasons for their impoverished and endangered state and the loss of Banaban ethnobotanical knowledge. The recorded flora of Banaba consists of approximately 205 species, of which only 50 are possibly native and none endemic. The balance of the flora is composed of ornamentals, weedy exotics, food plants, and a limited number of other useful cultigens. Although greatly outnumbered by exotics, indigenous species still dominate most areas including some of the most disturbed habitats, as well as constituting the most culturally and ecologically important resources on the island. Most plants had traditional native names and associated cultural knowledge, many serving as “trees of life.” It is argued that the protection and enhancement of the native and long-established non-indigenous flora and associated knowledge are crucial to the ecological integrity and survival of the people of Banaba and the culture and traditions of the Banaban people, most of who now live on Rabi Island in Fiji or elsewhere overseas.