Clonus is a series of involuntary, rhythmic, muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many cases is, accompanied by spasticity (another form of hyperexcitability).

Clonus is typically seen in people with cerebral palsy, stroke, multiple sclerosis, spinal cord damage and hepatic encephalopathy.

Clonus has also appeared after ingesting potent serotonergic drugs, where ingestion strongly predicts imminent serotonin toxicity (serotonin syndrome).