HMX, also called octogen or cyclotetramethylene-tetranitramine, is a powerful, but shock sensitive, nitroamine high explosive, chemically related to RDX.

First made in 1930, it is used almost exclusively in military applications including as the detonators in nuclear weapons and as a solid rocket propellant. It is usually blended with TNT or some other material to reduce its sensitivity.

Its molecule is an eight-membered ring made of four carbon and four nitrogen atoms, with a nitro group hanging off of each nitrogen atom. Because of its high molecular weight, it is one of the most powerful chemical explosives manufactured, although a number of newer ones, including HNIW and octanitrocubane are more powerful and less sensitive.

There are other names occasionally used for it — octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, tetrahexamine tetranitramine, or cyclotetramethylene tetranitramine. It may be produced by nitration of hexamine in the presence of acetic anhydride, paraformaldehyde and ammonium nitrate. RDX produced using the Bachmann Process usually contains 8-10% HMX.

Its CAS Number is 2691-41-0. The chemical formula is C₄H₈N₈O₈. It is a colorless solid with a melting point of 276 to 286 °C, although it usually decomposes at 280 °C. Its molecular weight is 296.20 and its density is 1.91 g/cm³. It is slightly soluble in water.

Like RDX, it has various mistranslations of its acronym including High Melting eXplosive, Her Majesty's eXplosive or even High velocity Military eXplosive. In fact the acronym simply means 'High Molecular weight rdX'.