Isoëtes tennesseensis, an octoploid species with a chromosome count of 2n = 88, is described. It occurs in the Hiwassee River in Tennessee. Past collections of this species have been misidentified as Isoëtes macrospora (= I. lacustris). Isoëtes tennesseensis differs from I. lacustris in chromosome number, megaspore and microspore morphology and distribution. Speculation on the origin of this new species is presented.