Two new polyhydroxylated macrolides, seimatopolides A (1) and B (2), were isolated from an EtOAc extract of Seimatosporium discosioides culture medium. The structures of the new compounds were established on the basis of spectroscopic analysis, including 1D and 2D NMR, and their absolute configurations were determined using the modified Mosher's method. Seimatopolides A (1) and B (2) activated peroxisome proliferator-activated receptor (PPAR)-γ with EC(50) values of 1.15 and 11.05 μM, respectively. The expression of PPAR-γ target genes in HepG2 hepatocytes was significantly altered; in particular, expression of the gluconeogenic genes glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK) was reduced upon stimulation with 1, supporting the proposal that compound 1 is both a PPAR-γ agonist and a possible therapeutic candidate for treatment of diabetes.