(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl or TEMPO is a chemical compound with the formula (CH₂)₃(CMe₂)₂NO. This heterocyclic compound is a red-orange, sublimable solid. As a stable radical, it has applications in chemistry and biochemistry. TEMPO was discovered by Lebedev and Kazarnowskii in 1960. It is prepared by oxidation of 2,2,6,6-tetramethylpiperidine. TEMPO is widely used as a radical marker, as a structural probe for biological systems in conjunction with electron spin resonance spectroscopy, as a reagent in organic synthesis, and as a mediator in controlled radical polymerization. The stability of this radical is attributed to the resonance provided by non-bonding electrons on the nitrogen atom, which form a 2c3e (half-) bond between nitrogen and oxygen, and hyperconjugative ability. Additional stability arises from the steric protection provided by the four  methyl groups adjacent to the nitroxyl group; however, the methyl groups prevent a double bond occurring between either carbon adjacent to nitrogen. The stability of the radical is also indicated by the weakness of the O-H bond in the hydrogenated derivative TEMPO-H. With an O-H bond dissociation energy of about 70 kcal/mol, this bond is about 30% weaker than a typical O-H bond.

In musical terminology, tempo ("time" in Italian; plural: tempi ) is the speed or pace of a given piece or subsection thereof.