Opuntia monacantha is a Cactaceae found in regions with few nutrients and water. This study aimed to identify bioactive compounds in the fruit of O. monacantha in southern region of Brazil. In the fruit of O. monacantha five phenolic compounds were identified: protocatechuic acid, ferulic acid, pcoumaric acid, catechin and quercetin. The ferulic acid and catechin were found in higher concentration in the fruit. The terpenes α-amyrin, β-amyrin, oleanolic acid and cycloartenol were also identified in the hexane fractions of the fruit. Unsaturated fatty acids were found in the fruit seed, with the highest concentration of linoleic acid (69%).