Ultra-hot Jupiters have temperatures at which molecules are thermally dissociated. The thermal dissociation of water leads to the production of the hydroxyl radical (OH). We analyzed high spectral resolution observations of a transit of WASP-76b obtained using CARMENES and searched for the presence of OH in its atmosphere. By cross-correlating with a model template we were able to detect OH in the atmosphere of the planet. This is the second detection of OH in an exoplanet atmosphere and can be used to trace thermal dissociation in ultra-hot Jupiters.