The outcomes of marine oil spills depend on the local conditions at the spill site, the length of the exposure and the sensitivity of relevant biological receptors, including tropical reef building corals. While oil toxicity towards marine organisms has been studied for decades, environmental risk assessments for activities involving oils generally only use limited data or generic threshold values; neither of which typically account for local environmental conditions (e.g. light spectrum and intensity) or consider exposure lengths. Here we present the first experimental data on the time- and light-dependent sensitivity of the tropical coral Acropora millepora to the water-soluble fraction of a commonly used heavy fuel oil (HFO). Fragments of adult corals were exposed in a novel flow-through experimental system to serially diluted, low energy water accommodated fractions of HFO over 14 days in the presence or absence of ecologically relevant ultraviolet radiation (UVR) intensities. Both exposure length and light spectrum significantly increased oil toxicity towards A. millepora adults, with partial mortality observed within less than 24 h in the presence of UVR. In contrast, mortality was not observed in the absence of UVR until after 96 h of exposure. Similarly, lateral growth was negatively affected at lower concentrations, and after shorter exposure periods, in the presence of UVR than under visible light alone. These experimental findings highlight the importance of accounting for local environmental conditions and exposure duration during environmental risk assessments and spill response planning.