We report experimental observations of a chemically bound dimer of 2,4,6-trinitrotoluene (TNT) produced by high-pressure shock waves. The experimental observations were made with a time-of-flight (TOF) mass spectrometer within which it is possible to produce strong shock waves by detonating condensed-phase explosives. The dimer is thought to arise from a Diels-Alder (DA) cross-linking of two TNT molecules. It is noteworthy that DA reactions are strongly pressure enhanced. We found that under some shock conditions a significant fraction of the TNT molecules are dimerized. The dimerization reaction, which is endothermic, may play a role in the shock insensitivity of TNT. Ancillary experiments in which TNT was evaporated and expanded through a nozzle into the mass spectrometer are also reported. It was possible in these experiments to produce a weakly bound TNT dimer in which the binding forces are those characteristic of a crystal. We show that this type of dimer has a different fragmentation pattern caused by electron impact ionization than the one produced by a shock wave. A rough estimate of the binding energy of this type dimer is given. The difference in the fragmentation patterns of the two types of dimer indicates that the dimer produced in the shock wave experiments is a physically different structure than the weakly bound species produced in the evaporation experiments. This supports the view that the shock-produced dimer is chemically bound. Two-dimensional time-dependent reactive hydrodynamic modeling of the shock wave experiments is used to produce estimates of the pressure and temperature fields in the shocked TNT as a function of space and time. The calculations, in conjunction with the experimental data, allow us to estimate the time scale of the shockwave-produced TNT dimerization reaction as being roughly 10 ns.