Objects on a collision course with an observer produce a specific pattern of optical expansion on the retina known as looming, which in theory exactly specifies time-to-collision. Looming stimuli produce stereotyped defensive responses in monkeys and human infants, indicating that the primate visual system is intrinsically tuned to interpret this stimulus as threatening. We investigated how emotional reactions to the semantic content of a looming stimulus affects perceived time-to-collision. We presented either threatening (snakes, spiders) or non-threatening (butterflies and rabbits) stimuli which expanded in size at a rate indicating one of five different times to contact (3.0, 3.5, 4.0, 4.5, or 5.0 seconds). After 1 second, the image disappeared and participants judged when it would have made contact with them continuing on the same trajectory. Consistent with previous findings, time–to-collision judgments systematically underestimated true arrival time. More importantly, our results showed that the magnitude of this underestimating significantly increased for threatening, compared to non-threatening, stimuli. Further, the magnitude of this increase was correlated with participants’ self-reported fear of snakes and spiders. Traditionally, looming has been interpreted as a purely optical cue to collision. Against that view, our results demonstrate that the semantic content of visual stimuli modulates perceived time-to-contact. Our results suggest that perceived threat led participants to use a larger margin of safety: underestimating time-to-collision errs on the side of allowing more time to prepare defensive responses (or flee). More generally, these results demonstrate that emotion has widespread effects on even very basic aspects of perception.