This study focused on the diet of Decken’s sifaka (Propithecus deckenii) and was carried out in Mandrozo Protected Area (MPA), in western Madagascar. The main objective was to identify the plants consumed by sifakas and their traditional uses by the local people within and surrounding MPA. For this purpose, we observed and recorded the feeding behavior of four groups of sifakas in two forest fragments (Andapabe and Ampiliravao), during two periods: from March to April 2018 for the wet season and from September to October 2020 for the dry season, using the Focal Animal Sampling method. Sifakas consumed 37 plant species belonging to 20 families and 31 genera and they ate mainly the plant leaves. We found that the sifaka’s diet is more diversified during wet season. We also interviewed 168 local persons from eight villages and recorded their ethnobotanical knowledge on the plant species eaten by sifakas. Local people utilized 70% (n=26) of the plant species eaten by sifakas. Five categories of human uses were recorded: house construction and tools for domestic use, phytotherapy, making of handicrafts, as food sources and traditional culture. Among these, the construction of houses and tools for domestic use and traditional medicine constitute the main mentioned human uses by the queried locals. For the indigenous people, three species of trees: Dalbergia humbertii, Tamarindus indica and Cedrelopsis grevei were used as part of their habits and customs.