Mesopelagic animals have a striking diversity of adaptations for vision in their dim habitat, and these have been investigated in most detail for chamber-eyed fish and compound-eyed crustaceans. Far less is known about the visual capabilities of mesopelagic species with naupliar eyes, including the abundant and ecologically important copepods. Here we present data on both the structure and function of the visual system in the metridinid copepod Gaussia princeps. This species is a vertical migrator, typically residing in oxygen minimum zones during the day and ascending into shallower (and warmer) waters at night to feed. We used (1) light and electron microscopy to describe the tripartite naupliar eye, (2) electroretinography to characterize temporal and spectral aspects of vision, and (3) contrast ratio modeling to evaluate the tuning of its spectral sensitivity for bioluminescence detection. The G. princeps eye is highly sensitive to light with 10.07 log photons cm-2 s-1 evoking half-maximum response amplitude in the VlogI curve. This sensitivity is due in part to slow photoreceptor dynamics; maximum critical flicker fusion frequencies are 7.2 – 12.0 Hz over the temperature range G. princeps experiences during its diel vertical migrations. Temporal resolution is moderately temperature sensitive (Q10 = 2.12 – 2.55), but insensitive to light adaptation. The spectral sensitivity functions in dark- and chromatic-adapted G. princeps eyes predict a single visual pigment at 496 nm. Gaussia princeps spectral sensitivity was better fit to its own luminescence emission spectrum than to the luminescence spectra of other representative mesopelagic species due to the presence of increased emission at wavelengths >500 nm in the G. princeps luminescence spectra. A signal:noise model supported that the observed G. princeps spectral sensitivity, which is offset to longer wavelengths relative to downwelling light, would serve to increase spectral contrast when viewing its luminescence against background mesopelagic light. Collectively, these data suggest the tripartite naupliar eye of G. princeps is well-adapted for vision in mesopelagic habitats, with slow photoreceptor dynamics and concomitant sensitivity, as well as spectral tuning for luminescence detection.