Herbs, annual or rarely perennial, not rhizomatous, caulescent; turions absent [present]. Leaves alternate to subopposite, submersed, sessile; sheath not persisting longer than blade, not leaving circular scar when shed, not ligulate, not auriculate; blade linear; intravaginal squamules scales, 2. Inflorescences terminal, capitate spikes, with subtending spathe, pedunculate; peduncle following fertilization often elongating, often spiraling. Flowers bisexual; subtending bracts absent; perianth absent; stamens 2, in 1 series; anthers distinct, dehiscing longitudinally; pollen arcuate; pistils 4--16, distinct, stipitate; ovules parietal, campylotropous. Fruits drupaceous. Seeds 1; embryo straight.

Ruppiaceae are found submersed in brackish or saline waters or fresh waters with very high calcium or sulfur ion concentrations.