Although the deep galleries of natural underground cavities are difficult to access and are sometimes dangerous, they have the potential to preserve trace fossils. Here, we report on the first occurrence of sauropod dinosaur tracks inside a karstic cave. Three trackways are preserved on the roof of the Castelbouc cave 500 m under the surface of the Causse Méjean plateau, southern France. The tracks are Bathonian in age (ca. 168–166 Ma), a crucial but still poorly known time interval in sauropod evolution. The three trackways yield sauropod tracks that are up to 1.25 m long and are therefore amongst the largest known dinosaur footprints worldwide. The trackmakers are hypothesized to be titanosauriforms. Some of the tracks are extremely well preserved and show impressions of digits, digital pads, and claws. We erect the new ichnogenus and ichnospecies Occitanopodus gandi, igen. et isp. nov. In order to characterize depositional environments, we conducted sedimentological, petrographic, and mineralogical analyses. The tracks from Castelbouc attest the presence of sauropods in proximal littoral environments during the Middle Jurassic. This discovery demonstrates the great potential of prospecting in deep karst caves that can occasionally offer larger and better-preserved surfaces than outdoor outcrops.