Gibberellins (GAs) and brassinosteroids (BRs), two growth-promoting phytohormones, regulate many common physiological processes. Their interactions at the molecular level remain unclear. Here, we demonstrate that OsGSR1, a member of the GAST (GA-stimulated transcript) gene family, is induced by GA and repressed by BR. RNA interference (RNAi) transgenic rice plants with reduced OsGSR1 expression show phenotypes similar to plants deficient in BR, including short primary roots, erect leaves and reduced fertility. The OsGSR1 RNAi transgenic rice shows reduced endogenous BR level, and the dwarf phenotype could be rescued by application of brassinolide. Yeast two-hybrid assay revealed that OsGSR1 interacts with DIM/DWF1, an enzyme that catalyzes the conversion from 24-methylenecholesterol to campesterol in BR biosynthesis. These results suggest that OsGSR1 activates BR synthesis by directly regulating a BR biosynthetic enzyme at the posttranslational level. Furthermore, OsGSR1 RNAi plants show reduced sensitivity to GA treatment, increased expression of the GA biosynthetic gene OsGA20ox2, which is feedback inhibited by GA signaling, and elevated level of endogenous GA, which together suggest that OsGSR1 is a positive regulator of GA signaling. These results demonstrate that OsGSR1 plays important roles in both BR and GA pathways and mediates an interaction between the two signaling pathways.