Lobelia siphilitica, the great blue lobelia or great lobelia, is a plant species within the Campanulaceae family. It is a herbaceous, perennial dicot native to eastern and central Canada and United States. Growing up to three feet tall, it lives in zones 4 to 9 in moist to wet soils. It produces a spike of zygomorphic flowers in the late summer.

It blooms from August to October. It is a short lived perennial (with each plant living for only a few years).

Although self-compatible, a flower is unable to offer pollen to itself and it must be pollinated by insects (primarily bees in the Bombus genus).