5-hydroxymethyl-2-furfural is selectively converted into 2,5-furandicarboxylic acid (99 mol % yield) in water, under mild conditions (65–130 °C, 10 bar air) with gold nanoparticles supported on nanoparticulate ceria (Au-CeO2) and on titania (Au-TiO2); the former being more active and selective. A reaction mechanism is established and the rate-limiting step of the reaction is the alcohol oxidation of 5-hydroxymethyl-2-furancarboxylic acid into 2,5-furandicarboxylic acid. The effects of pressure, temperature, substrate-to-catalyst ratio, and amount of base are studied to find the most suitable reaction conditions. The absence of metal leaching is verified by chemical analysis of the reaction solution and by confirming that the reaction does not occur after catalyst removal by filtration in hot water. Substrate degradation is strongly diminished and the catalyst life increased by performing the reaction in two temperature steps.