Polarization sensitivity has most often been studied in mature insects, yet it is likely that larvae also make use of this visual modality. The aquatic larvae of the predacious diving beetle Thermonectus marmoratus are highly successful visually guided predators, with a UV-sensitive proximal retina that, according to its ultrastructure, has three distinct cell types with anatomical attributes that are consistent with polarization sensitivity. In the present study we used electrophysiological methods and single-cell staining to confirm polarization sensitivity in the proximal retinas of both principal eyes of these larvae. As expected from their microvillar orientation, cells of type T1 are most sensitive to vertically polarized light, while cells of type T2 are most sensitive to horizontally polarized light. In addition, T3 cells probably constitute a second population of cells that are most sensitive to light with vertical e-vector orientation, characterized by shallower polarization modulations, and smaller polarization sensitivity (PS) values than are typical for T1 cells. The level of PS values found in this study suggests that polarization sensitivity probably plays an important role in the visual system of these larvae. Based on their natural history and behavior, possible functions are: (1) finding water after hatching, (2) finding the shore before pupation, and (3) making prey more visible, by filtering out horizontally polarized haze, and/or using polarization features for prey detection.