Lobelia siphilitica, the great blue lobelia, great lobelia, or blue cardinal flower is a plant species within the family Campanulaceae. 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.

Lobelia siphilitica is commonly called Great Blue Lobelia as it has larger flowers than most other blue flowered Lobelia spp. Lobelia siphilitica is a clump forming perennial wildflower with toothed lance shaped leaves. In late summer stems are topped by spiky racemes of intense blue-violet florets.