Tetraethyl pyrophosphate, abbreviated TEPP, is an organophosphate compound with the formula [(C₂H₅O)₂P(O)]₂O. It is the tetraethyl derivative of pyrophosphate (P₂O₇^4-). It is a colorless oil that solidifies near room temperature. It is used as an insecticide. The compound hydrolyzes rapidly.

TEPP is an insecticide to aphids, mites, spiders, mealybugs, leafhoppers, lygus bugs, thrips, leafminers, and many other pests. TEPP and other organophosphates are the most widely used pesticides in the U.S. due to their effectiveness and relative small impact on the environment because this organophosphate breaks down so easily.^{[citation needed]}

TEPP has been used for treatment for myasthenia gravis, an autoimmune disease. The treatment would deliver an increase in strength.

The synthesis by De Clermont and Moschnin was based on the earlier work by Alexander Williamson (who is well known for the Williamson ether synthesis). Their synthesis made use of ethyl iodide and silver salts to form esters in combination with pyrophosphate.

Commercial routes to TEPP often use methods developed by Schrader, Woodstock, and Toy. Triethyl phosphate reacts with phosphorus oxychloride (Schrader's method) or phosphorus pentoxide (Woodstock's method). Alternatively, controlled hydrolysis of diethyl phosphorochloridate delivers the compound:

The related tetrabenzylpyrophosphate is prepared by dehydration of dibenzylphosphoric acid:

TEPP and most of the other organophosphates are susceptible to hydrolysis. The product is diethyl phosphate.

TEPP is bioactive as an acetylcholinesterase inhibitor. It reacts with the serine hydroxyl group at the active site, preventing this enzyme from acting on its normal substrate, the neurotransmitter acetyl choline.^{[citation needed]}