Lobelia cardinalis, the cardinal flower is a species of flowering plant in the bellflower family Campanulaceae native to the Americas.

Lobelia cardinalis is related to two other Lobelia species in to the Eastern United States, Lobelia inflata (Indian tobacco) and Lobelia siphilitica (great lobelia) which all display the characteristic "lip" petal near the opening of the flower and the "milky" liquid the plant excretes. L. siphilitica has blue flowers and is primarily pollinated by bees, whereas L. cardinalis is red and is primarily pollinated by the ruby-throated hummingbird.

Lobelia was introduced to Europe in the mid-1620s, where the name cardinal flower was in use by 1629, likely due to the similarity of the flower's color to the vesture of Roman Catholic Cardinals who had formerly wore Tyrian purple, but switched to the colour scarlet when purple dye sources became scarce.