The transcription factor estrogen receptor β (ERβ) plays roles in the central nervous, endocrine, cardiovascular, and immune systems. ERβ can be SUMOylated. However, the underlying mechanism remains unclear. Here, we show that RSRC1/SRrp53 interacts with ERβ and SUMOylation of RSRC1 is required for regulation of PIAS1-mediated ERβ SUMOylation. RSRC1 promotes ERβ SUMOylation through enhanced interaction between ERβ and PIAS1. RSRC1 represses ERβ transcriptional activity through regulation of ERβ SUMOylation. By establishing RSRC1 as a novel cofactor for SUMOylation, our data provide insight into regulation of ERβ SUMOylation and indicate that SUMOylation of one protein can regulate another protein SUMOylation.