大海牛（学名：Hydrodamalis gigas），又名巨儒艮、无齿海牛或斯特拉海牛，是已灭绝的巨大哺乳类动物。牠们是海牛目中已知体型最大的物种，比起其近亲海牛与儒艮都要来得大，而牠们也是生存至近代的海牛目动物中，唯一适应寒带气候的物种。大海牛于1741年在白令海峡发现，发现者为博物学者乔治·斯特拉，当时他正与探险家维他斯·白令一同旅行，而他也是已知唯一看过大海牛活体的生物学家。当时大海牛的数目已十分少，主要生活在靠近科曼多尔群岛白令岛与铜岛（медний）的近海水域。

The Steller's sea cow (Hydrodamalis gigas) is an extinct herbivorous marine mammal. It was the largest member of the order Sirenia, which includes its closest living relative, the dugong (Dugong dugon), and the manatees (Trichechus spp.). It reached up to 9 metres (30 ft) in length, making it among the largest mammals other than whales to have existed in the holocene epoch. Although the sea cow had formerly been abundant throughout the North Pacific, by 1741, when it was first described by Georg Wilhelm Steller, chief naturalist on an expedition led by explorer Vitus Bering, its range had been limited to a single, isolated population surrounding the uninhabited Commander Islands. Within 27 years of discovery by Europeans, the slow-moving and easily captured Steller's sea cow was hunted to extinction.