The semicircular canals (SCC) and the otoliths are the two main organs of the vestibular system responsible for balance and gaze-stabilization. Whereas the otoliths as gravity detectors are largely expected to be impacted, the effect of weightlessness on the SCC is much less well understood. Despite the number of studies that have investigated the effect of microgravity on the SCC through horizontal vestibulo-ocular reflex (hVOR) measurements in the past decades, most of those studies were strongly limited by the restricted sample size and the short mission duration. This study aims to characterize the hVOR changes before and after a long-duration spaceflight (>6 months) in an unprecedently large database of 44 cosmonaut experiments. We found a significant decrease in hVOR time constant early postflight (R+1/4) and late postflight (R+9/12) compared to preflight. A partial but not complete recovery was seen 9 to 12 days after their return. These findings reveal that the SCC function is altered as a result of spaceflight.