In this work, carbon nanotubes (CNTs) supported rhodium nanoparticles (Rh NPs) (Rh(0)/CNTs) have been successfully prepared via a very simple in situ method and used as the catalysts for hydrogen generation from the hydrolysis of ammonia borane (AB, NH3BH3) under ambient atmosphere at room temperature. The as-synthesized nanocatalysts were characterized by XRD, SEM, TEM, EDX, ICP, and XPS measurements. The characterized results revealed that ultrafine Rh NPs of about 1.1-3.4 nm were well-dispersed on CNTs. Compared with free Rh(0) NPs and Rh(0)/activated charcoal, the Rh(0)/CNTs showed higher catalytic activity for the hydrolysis of NH3BH3 with a total turnover frequency (TOF) value up to 706 mol H2 (mol Rh min)−1, which was among the highest of all the heterogeneous catalysts ever reported for the same reaction. The recyclability tests revealed that the Rh(0)/CNTs are still active after five runs of hydrolysis of NH3BH3 providing 100% conversion and retaining 61% of their initial catalytic activity (TOF = 431 mol H2 (mol Rh min)−1). In addition, the activation energy for the hydrolysis of NH3BH3 catalyzed by Rh(0)/CNTs was measured to be 32 ± 1 kJ/mol, which was lower than most of the values reported for other catalysts.