Diademed sifakas are mostly folivorous and feed on at least 100 different identified plant species. Not much is known yet of their digestive system, but they do have large salivary glands. It is thought that these large glands may deactivate tannins with compounds in their saliva. In normal conditions, diademed sifakas spend half of their feeding time on immature leaves and a quarter on fruit. Even so, leaves are their least favorite food item on the menu; they much prefer fruit, seeds, buds, and flowers. They also consume soil, galls (a growth on the external tissues of plants), leafstalks, bark, insects, and anything else that might strike their fancy. There is no conclusion yet as to why they occasionally ingest soil, but there are some theories: it may help detoxify plant toxins or tannins; it may supplement their diet with trace minerals; and it may help them fend off parasites.

During the rainy season (from November to March), fruit is abundant and sifakas living in continuous forested areas get about 70 percent of their food foraging in three tree species (Ocotea, Erythroxylum, and Maintipototra). Between June and October (in the early to late dry season), leaves become their main food source.

Time spent foraging varies greatly between groups living in fragmented forest compared to continuous forest. The former spend 30-40% of their time feeding on mistletoe, as opposed to 28-30% for the latter. It is also important to note that in fragmented forests, individuals feed on multiple patches and dominant individuals usually feed on the best patches, so it is probable that lower-ranking individuals get less nutrients in fragmented forest than they would get in continuous forest. Studies indicate that similarly aged immature sifakas weigh less in fragmented forest than in continuous forest.

Mistletoe is a year-long staple and groups living in fragmented forests heavily rely on it—except in January when they are able to find seeds from fruiting tree species like Macaranga ankafinensis.

Primates usually forage for food using sight, by spotting ripe fruit and memorizing those locations. Diademed sifakas also rely on olfactory senses, specifically to find Langsdorffia and Cytinus. These plants grow on the roots of trees and vines. They only emerge from the ground when mature and have a strong sweet smell when split open. Between the months of July and October, hungry sifakas descend from the trees, walk slowly while sniffing the ground left and right, and dig and overturn leaves until they find the delicacy they are looking for. Foraging sifakas are totally focused on their task, not even bothering to play; since cases of aggressive behavior and theft have been observed during the search for some shoots and roots, this likely indicates that they are prized food items.

Sifakas are also unique compared to other primates because during the lean season, instead of traveling longer distances, spending more time foraging and eating more, they travel less, spend less time foraging, and eat less food. Why they do this is not well understood. It could be that preferred food items are easy to pick, easy to ingest, easy to digest, and provide good nutrition, whereas fallback foods like mistletoe, although abundant, provide fewer calories and little nutritional gain, are harder to digest, and therefore require more energy.