Lobelia oahuensis is a rare species of flowering plant in the bellflower family known by the common name Oahu lobelia. It is endemic to Hawaii, where it is known only from the island of Oahu. There are about 100 individuals remaining in the Koʻolau Range, and only one known individual in the Waianae Range. It is federally listed as an endangered species of the United States. The inflorescence of this plant is over a meter long and densely packed with blue flowers.