Tetrahydropiperine, the first natural aryl pentanamide, was isolated from the fruits of Piper longum. The dried fruits were powdered and extracted with CH2Cl2–MeOH (1:1), followed by column chromatography over silica gel to obtain tetrahydropiperine. Its structure was determined by spectral data (1H NMR, 13C NMR, MS). This is the first report of an arylpentanamide from a natural source. The investigation indicates that enzymes responsible for the biochemical transformation (hydrogenation) of arylpentdienamides or arylpentenamides to arylpentanamides exist in nature, but their frequency, activity, and/or action may be very low.