Garcinia schomburgkiana​ is a medium-sized tree found in Thailand, Laos, Cambodia, and Vietnam as an endemic plant. This plant has long been used as folk medicine. In this paper, the phytochemical study of Garcinia schomburgkiana branches collected in Thailand leads to the isolation of thirteen known compounds, comprising two polyprenylated benzoylphloroglucinols, seven xanthones, two chalcones, one benzophenone, and one biphenyl, and the evaluation of their cytotoxic properties and a-glucosidase inhibitory activities. The isolated compounds are 7-epiclusianone (1), oblongifolin C (2), pancixanthone A (3), euxanthone (4), 1,4,5-trihydroxyxanthone (5), osajaxanthone (6), 1,2,7-trihydroxyxanthone (7), 1,3,7-trihydroxy-2,4-diprenylxanthone (8), cudratricusxanthone K (9), 2'-hydroxydihydrochalcone (10), 1-(2',6'-dihydroxyphenyl)-3-phenyl-1-propanone (11), clusiacitran B (12), and aucuparin (13). This study applied the analysis of all isolated compounds’ NMR spectroscopic data (¹H and ¹³C NMR) with the literature to enact their chemical structures. Moreover, this investigation yields three compounds that have never been discovered from the genus Garcinia, namely cudratricusxanthone K, 2'-hydroxydihydrochalcone, and 1-(2'-6'-dihydroxyphenyl)-3-phenyl-1-propanone). The cytotoxic and a-glucosidase inhibitory activities were tested by using the MTT assay and rat small intestine method, respectively. The evaluation of the cytotoxic properties towards Hep G2, MCF-7, HeLa S-3, KB, and HT-29 cell lines showed that oblongifolin C possessed the strongest activities with IC₅₀ values ranging from 5.15 to 7.09 mM. Meanwhile, among all of the compounds tested for a-glucosidase inhibitions, aucuparin showed moderate inhibitory effects in sucrose and maltose substrates with the IC₅₀ values of 49.23 mM and 98.20 mM, respectively.