Nail biting, also known as onychophagy or onychophagia, is an oral compulsive habit of biting one's fingernails. It is sometimes described as a parafunctional activity, the common use of the mouth for an activity other than speaking, eating, or drinking.

Nail biting is very common, especially amongst children. 25–30 percent of children bite nails. More pathological forms of nails biting are considered an impulse control disorder in the DSM-IV-R and are classified under obsessive-compulsive and related disorders in the DSM-5. The ICD-10 classifies the practice as "other specified behavioral and emotional disorders with onset usually occurring in childhood and adolescence". However, not all nail biting is pathological, and the difference between harmful obsession and normal behavior is not always clear. The earliest reference to nail biting as a symptom of anxiety was in late sixteenth century in France.