Background: Many exopolysaccharides from the endophytes in medicinal plants possess various potential bioactivities. Materials and Methods: The endophytic fungus JY25 was isolated from the leave of the Chinese medicinal plant Gynostemma pentaphylla and identified as Chaetomium sp. by its phylogenetic and physiological analysis. One exopolysaccharide (EPS) fraction was isolated from the fermentation broth by ethanol precipitation and purified by gel filtration chromatography on Sepharose CL-6B. The molecular characteristics were examined by GC-MS, FT-IR, and multiangle laser light scattering (MALLS). Results: The monosaccharide composition analysis indicated that the purified EPS was mainly composed of glucose, mannose, arabinose, and galactose with the molecular ratio of 78.29: 8.99: 8.64: 4.08. FT-IR spectral analysis of the purified EPS revealed prominent characteristic groups, such as carbonyl bond, pyranose ring, and so on. The weight-average molar mass and the polydispersity ratio of the EPS were revealed to be 1.961×104 g/mol and 1.838, respectively. Furthermore, thermo gravimetric analysis (TGA) indicated that the degradation temperature of the purified EPS was 305°C. The purified EPS from the endophytic fungus Chaetomium sp. displayed antioxidant and antiproliferative activities. Conclusion: The results demonstrated that the EPS could be used as a healthful food and material source in pharmaceutical industries.