Ammonia can be used as fuel in internal combustion engines (ICEs). In this case, a flame accelerator, such as hydrogen, is needed. H2 can be produced on-board by partial decomposition of ammonia. In this work, ruthenium nanoparticles were embedded into a lanthanum-stabilized zirconia (LSZ) support to obtain active and stable heterogeneous catalysts for NH3 decomposition. The effects of the preparation of both Ru nanoparticles and LSZ support were investigated. The embedded catalysts present high metal dispersion and good metal accessibility. Despite the relatively low metal loading (3 wt %), activity was very high in the temperature range 400–600 °C. The activity of the reference catalysts prepared by using classical impregnation was significantly lower under the same working conditions. Although many factors contribute to the final catalyst performances, the data reported confirm that the embedding strategy minimizes the undesirable sintering of the Ru nanoparticles, leading to promising and stable catalytic activity.