The hydrogen trapping character of NbC nano-particles in a 0.05C-0.41Nb-2.0Ni tempered martensitic steel was investigated by hydrogen thermal-desorption spectrometry (TDS) analysis and atomic-level microstructural observation. The steel trapped the most hydrogen after tempering at 600 °C, and the trapped hydrogen decreased with increasing tempering temperature. The precipitation of NbC nanoparticles is responsible for the sharp increase at 600 °C. The coarsening of precipitates reduces the amount of trapped hydrogen. The TDS analysis of the same specimen charged with various levels of hydrogen concentration indicated that the apparent activation energy for desorption from NbC precipitates is dependent on hydrogen concentration. Hydrogen trap sites are discussed in terms of the character of the NbC/matrix interface and the crystal defects present at and around the interface.