Simultaneous hydrodesulfurization (HDS), hydrodearomatization (HDA), and hydrodenitrogenation (HDN) reactions have been studied over sulfided RuNiMo/Al2O3 catalysts. The effects of Ru promotion and alumina support modification with HY zeolite and P on catalyst performance have been studied using a synthetic feed containing dibenzothiophene, toluene, naphthalene, and pyridine. Activity tests were carried out in a semi-automatic microplant equipped with a continuous flow reactor, working under conditions similar to industrial practice (P = 5 MPa, T = 285−360 °C, and liquid hourly space velocity (LHSV) = 3 and 4.5 h−1). The catalysts were characterized by SBET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia (TPD−NH3), and X-ray photoelectron spectroscopy (XPS) techniques. Activity tests revealed that optimal Ru loading is 1 wt % (nominal). Under fixed experimental conditions (300 °C, 5 MPa, and LHSV = 3 h−1), ultra-low sulfur diesel (S < 20 ppm S) was obtained over a Ru(1%)Ni(5%)Mo(14%)/γ-Al2O3 catalyst from a feed containing 18361 ppm S. However, this catalyst showed similar capability toward aromatics and pyridine removal as a commercial NiMo/Al2O3 catalyst. At a reaction temperature of 345 °C, the 5% HY−Al2O3 substrate led to a small increase in the hydrogenation capability of this catalyst but without enhancement of its HDS/HDN capability. On the contrary, the P-modified substrate resulted in a lower activity. The properties of the supported catalysts are discussed in terms of the support effect and Ru promotion.