The urbanization of the shores of the Gulf of Aqaba has exposed the marine environment there, including unique fringing coral reefs, to strong anthropogenic light sources. Here we present the first in situ measurements of artificial nighttime light under water in such an ecosystem, with irradiance measured in 12 wavelength bands, at 19 measurement stations spread over 44 square km, and at 30 depths down to 30-m depth. At 1-m depth, we find downwelling irradiance values that vary from 4.6 × 10-4 μW cm-2 nm-1 500 m from the city to 1 × 10-6 μW cm-2 nm-1 in the center of the gulf (9.5 km from the city) in the yellow channel (589-nm wavelength) and from 1.3 × 10-4 μW cm-2 nm-1 to 4.3 × 10-5 μW cm-2 nm-1 in the blue channel (443-nm wavelength). Down to 10-m depth, we find downwelling irradiance values that vary from 1 × 10-6 μW cm-2 nm-1 to 4.6 × 10-4 μW cm-2 nm-1 in the yellow channel and from 2.6 × 10-5 μW cm-2 nm-1 to 1.3 × 10-4 μW cm-2 nm-1 in the blue channel, and we even detected a signal at 30-m depth. This irradiance could influence such biological processes as the tuning of circadian clocks, the synchronization of coral spawning, recruitment and competition, vertical migration of demersal plankton, feeding patterns, and prey/predator visual interactions.