巨獭（学名Pteronura brasiliensis），又被称为巴西大水獭，是一种水栖食肉哺乳类，原产于南美，目前已经濒临灭绝。

雄性可长达1.5至1.8公尺，雌性可达1.5至1.7公尺，早期的毛皮标本报告甚至有2.4公尺雄性的记录，是世界上身体最长的水獭。密集的捕杀已经降低这种巨型个体的发生率。雄性体重约26至32公斤，雌性约22至26公斤，只有海獭可能比其重。

巴西大水獭是一种群居的动物，一般由一对夫妇养育5只左右的幼子，其中已成年的幼子中的一只（通常为雌性）担负保姆的作用。而成年的雄性大水獭这会在一到两年后离开种群，自己寻找栖息地。大水獭一般进行集体捕食，在一群大水獭的面前，世界上最大的蛇水蚺也不是其对手。大水獭唯一的天敌是体长到6米的成年凯门鳄，但是由于两者生活习性的差别，白天活动的大水獭其实很少受到夜间活动的鳄鱼的攻击，同时大水獭灵活的动作也是动作迟缓的鳄鱼必须克服的难点；同时幼年的鳄鱼甚至会成为大水獭的美食。 大水獭是一种生性好奇而且非常聪明的动物。形态上，巨獭下颚、咽部喉部具有不规则白斑。

The giant otter (Pteronura brasiliensis) is a South American carnivorous mammal. It is the longest member of the Mustelidae, or weasel family, a globally successful group of predators, reaching up to 1.7 m (5.6 ft). Atypical of mustelids, the giant otter is a social species, with family groups typically supporting three to eight members. The groups are centered on a dominant breeding pair and are extremely cohesive and cooperative. Although generally peaceful, the species is territorial, and aggression has been observed between groups. The giant otter is diurnal, being active exclusively during daylight hours. It is the noisiest otter species, and distinct vocalizations have been documented that indicate alarm, aggressiveness, and reassurance.