Tropical ecosystems hold an extremely diverse array of endophytic fungi, but their potential use still remains to be explored. In this study, we isolated an endophytic fungus from the leaves of Otoba gracilipes, a medicinal tree from a tropical rainforest in Colombia. Following isolation and cultivation, we evaluated its extracellular crude extract for antioxidant activity. Using traditional and molecular methods (ITS1, NL1 regions), the endophyte was identified as Fusarium oxysporum. Fresh spores from the fungal isolate were inoculated in liquid media (potato dextrose broth [PDB] and potato dextrose-yeast extract broth [PDYB]) and centrifuged for recovering extracellular polysaccharides from the exhausted medium after 30 days of cultivation. Crude extracts were recovered, purified, lyophilized, and evaluated for their ability to inactivate the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The extracts obtained from PDB culture media had a 51.5% of scavenging effect on DPPH after 5 min of reaction compared with the extracts from PDBY (26.4%), which suggests a high antioxidant potential of these fungal extracts. Thus, our results suggest other fungi from tropical ecosystems should be explored as potential sources of novel enzymes and other metabolites with bioactivity.