Newly emerged sea turtle hatchlings use a positive phototaxis at night to locate the ocean from their nest site (“seafinding” orientation). Photic energy is absorbed by dune vegetation but reflected from the ocean surface, providing a reliable intensity cue for this response. However, the light wavelengths mediating this response are unknown in leatherbacks (Dermochelys coriacea), a critically endangered species. We evoked a phototaxis under lab conditions by presenting hatchlings with near-monochromatic light wavelengths at intensities they would encounter at night. Our goal was to determine if leatherback spectral sensitivities to those wavelengths differed from those of related “hard-shelled” (green turtle, loggerhead) hatchlings, and if so how those differences affected seafinding behavior under natural conditions. We found that leatherbacks were (i) about an order of magnitude less sensitive to the same light wavelengths used by the other species, and that (ii) those sensitivity differences were correlated with more circuitous orientation paths shown by leatherbacks during their seafinding crawl. Those contrasts were reduced under lunar illumination that heightened intensity contrasts between the landward and seaward view. We conclude that while the two groups of species are equally adept at locating the sea from the nest, and while they favor detection of the same light wavelengths, sensitivity differences may be responsible for contrasts in their orientation accuracy. Those contrasts may also make leatherbacks more susceptible to the disruptive effects of increasingly common artificial lighting on seafinding during this crucial early phase of migration.