In this work, the catalytic performance of a synthesized dispersed Ni-based catalyst amorphous catalytic system of Ni­(NO3)­(OH) in the hydrothermal upgrading of high-sulfur Cuban extra-heavy oil at temperatures of 200 and 300 °C for a duration of 24 h was evaluated. The synthesized Ni-catalyst predominantly exhibits an amorphous structure, which is primarily attributed to nickel hydroxonitrate (Ni­(OH)­(NO3)), as verified through X-ray diffraction and thermogravimetric Fourier transform infrared analyses. The results show that the presence of the synthesized catalyst in the upgrading system improves the physical and chemical properties of the upgraded oil samples compared to non-catalytic aquathermolysis results. The application of the Ni-based catalyst resulted in a reduction of sulfur content by 9.0–17.4%, a decrease in viscosity by 11.4–11.7%, and a lowering of the heavy oil fraction by 6.8–8.3%. Additionally, there was an increment in the production of gases by 0.2–0.3% and an increase in the fraction of hydrocarbons with a carbon number of less than < C20 by 5.4–8.2%. The best upgrading performance was observed during catalytic aquathermolysis at 300 °C. Overall, the findings underscore the synthesized Ni-based catalyst’s potential to significantly enhance the in situ hydrothermal upgrading process of extra-heavy crude oil, offering a promising approach for the treatment of such challenging feedstocks.