We have detected the tert-butyl hydroperoxide dimer, (t-BuOOH)2, in the gas phase at room temperature using conventional FTIR techniques. The dimer is identified by an asymmetric absorbance band assigned to the fundamental hydrogen-bound OHb-stretch. The weighted band maximum of the dimer OHb-stretch is located at ∼3452 cm–1, red-shifted by ∼145 cm–1 from the monomer OH-stretching band. The gas-phase dimer assignment is supported by Ar matrix isolation FTIR experiments at 12 K and experiments with a partially deuterated sample. Computationally, we find the lowest energy structure of (t-BuOOH)2 to be a doubly hydrogen bound six-membered ring with non-optimal hydrogen bond angles. We estimate the gas-phase constant of dimer formation, K, to be 0.4 (standard pressure of 1 bar) using the experimental integrated absorbance and a theoretically determined oscillator strength of the OHb-stretching band.