Nearly 90% of orangutans' diet is composed of fruit. They eat more than 400 different plant varieties. Orangutans favor a fruit called durians. Durians are covered in sharp spines and have a pungent smell. Orangutans use a variety of tools and their powerful jaws to break into such fruits. Refer to tool use section.

Although fruit comprises most of the orangutan's diet, they still require other nutrients as part of their daily intake. They receive a mixture of sugars and fats from fruit, carbohydrates from leaves, and protein from nuts.

Orangutans spend up to six hours a day eating or foraging for food.

As fruit—eating animals, orangutans are important propagators of tropical plants. Many fruit seedlings sprout only after having passed through an animal's digestive system. Therefore, orangutans have a vital ecological role as seed dispersers in their rainforest environment and affect forest regeneration and plant—species diversity.

Orangutans strip leaves from branches by dragging them through their mouths.

Orangutans are capable of memorizing the locations of temporary food sources, tracking seasonal changes in fruiting peaks and identifying behavioral signs from other animals to find fruiting trees.

To help adsorb and neutralize secondary plant metabolites (plant compounds that are not directly involved in growth, development or reproduction) orangutans consume soil.

Orangutans get most of their water from the succulent fruits they eat, but will also drink from rivers and streams. Mother orangutans sometimes let their young drink from their mouths.

Orangutans are capable of opening hard—shelled and/or thorny fruits by gripping it firmly between their teeth and using one hand to manipulate it until a weak point is found, thereby breaking open the hard exterior. This feeding method enables orangutans to feed on certain vegetation before its ripe enough for other species such as elephants and various insects to eat.