Acetone peroxide (/æsəˈtəʊn pɛrˈɒksaɪd/ ^ⓘ also called APEX and mother of Satan) is an organic peroxide and a primary explosive. It is produced by the reaction of acetone and hydrogen peroxide to yield a mixture of linear monomer and cyclic dimer, trimer, and tetramer forms. The monomer is dimethyldioxirane. The dimer is known as diacetone diperoxide (DADP). The trimer is known as triacetone triperoxide (TATP) or tri-cyclic acetone peroxide (TCAP). Acetone peroxide takes the form of a white crystalline powder with a distinctive bleach-like odor when impure, or a fruit-like smell when pure, and can explode powerfully if subjected to heat, friction, static electricity, concentrated sulfuric acid, strong UV radiation, or shock. Until about 2015, explosives detectors were not set to detect non-nitrogenous explosives, as most explosives used preceding 2015 were nitrogen-based. TATP, being nitrogen-free, has been used as the explosive of choice in several terrorist bomb attacks since 2001.

Acetone peroxide (specifically, triacetone triperoxide) was discovered in 1895 by the German chemist Richard Wolffenstein. Wolffenstein combined acetone and hydrogen peroxide, and then he allowed the mixture to stand for a week at room temperature, during which time a small quantity of crystals precipitated, which had a melting point of 97 °C (207 °F).

In 1899, Adolf von Baeyer and Victor Villiger described the first synthesis of the dimer and described use of acids for the synthesis of both peroxides. Baeyer and Villiger prepared the dimer by combining potassium persulfate in diethyl ether with acetone, under cooling. After separating the ether layer, the product was purified and found to melt at 132–133 °C (270–271 °F). They found that the trimer could be prepared by adding hydrochloric acid to a chilled mixture of acetone and hydrogen peroxide. By using the depression of freezing points to determine the molecular weights of the compounds, they also determined that the form of acetone peroxide that they had prepared via potassium persulfate was a dimer, whereas the acetone peroxide that had been prepared via hydrochloric acid was a trimer, like Wolffenstein's compound.

Work on this methodology and on the various products obtained, was further investigated in the mid-20th century by Milas and Golubović.

The chemical name acetone peroxide is most commonly used to refer to the cyclic trimer, the product of a reaction between two precursors, hydrogen peroxide and acetone, in an acid-catalyzed nucleophilic addition, although monomeric and dimeric forms are also possible.

Specifically, two dimers, one cyclic (C₆H₁₂O₄) and one open chain (C₆H₁₄O₄), as well as an open dihydroperoxide monomer (C₃H₈O₄), can also be formed; under a particular set of conditions of reagent and acid catalyst concentration, the cyclic trimer is the primary product. Under neutral conditions, the reaction is reported to produce the monomeric organic peroxide.

A tetrameric form has also been described, under different catalytic conditions, albeit not without disputes and controversy.

The most common route for nearly pure TATP is H₂O₂/acetone/HCl in 1:1:0.25 molar ratios, using 30% hydrogen peroxide. This product contains very little or none of DADP with some very small traces of chlorinated compounds. Product that contains large fraction of DADP can be obtained from 50% H₂O₂ using large amounts of concentrated sulfuric acid as catalyst or alternatively with 30% H₂O₂ and massive amounts of HCl as a catalyst.