The Amazonian manatee is a herbivore that feeds on aquatic vegetation near lake edges, such as aquatic grasses, and floating vegetation such as water lilies. Captive adults daily consume from 9 – 15 kilograms (20 – 33 pounds) of leafy vegetables.

It occurs exclusively in fresh water. It prefers blackwater lakes, oxbows and lagoons and it has been maintained successfully in waters with temperatures of 22 – 30 deg C (72 – 86 deg F). The Amazonian manatees key requirements in the wild seem to be large blackwater lakes or lagoons with deep connections to large rivers and abundant aquatic vegetation.

The Amazonian manatee is both diurnal (active during the day) and nocturnal (active during the night). It is entirely aquatic and never leaves the water. Amazonian manatees do most of their feeding during the wet season, when they eat new vegetation in seasonally flooded backwaters.

During the dry season (September – March), when they congregate in the main river channels or in deep parts of larger lakes, they may fast for weeks or months for lack of available food plants.