Abstract Design of highly active and stable catalysts for ammonia decomposition to produce CO x ‐free hydrogen is an urgent task for the utilization of NH 3 as H 2 carrier. Herein, Yttrium oxide (Y 2 O 3 ) has been demonstrated to be capable of dispersing and stabilizing Ru nanoparticles as a functional support. Highly dispersed Ru nanoparticles on Y 2 O 3 (Ru/Y 2 O 3 ) was prepared and investigated as catalyst for NH 3 decomposition. Under a relatively high weight gas hourly space velocity (30000 mL g cat −1 h −1 ), the Ru/Y 2 O 3 with Ru content of 5 wt. % can catalyze NH 3 decomposition with excellent catalytic activity and can achieve nearly complete NH 3 conversion at 475 °C, superior to many typical oxides supported Ru catalysts. The Ru/Y 2 O 3 catalyst is also highly stable for NH 3 decomposition, and the activity can be well maintained in a period of 100 hours. Characterization results reveal that the superior catalytic performance of Ru/Y 2 O 3 for NH 3 decomposition reaction may originate from the high dispersion of Ru nanoparticles as well as their strong interaction with Y 2 O 3 support. This work offers opportunity to achieve new kind of active and stable oxide supported Ru catalysts for NH 3 decomposition reaction.