RXR belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. Here we demonstrate that RXR is a target substrate of a SUMO-specific protease, SUSP1 that is capable of controlling the transcriptional activity of RXR. RXR was modified by SUMO-1 in vivo as well as in vitro, and the Lys108 residue within the IKPP sequence of RXR AF1 domain was identified as the major SUMO-1 acceptor site. Prevention of SUMO modification by Lys-to-Arg mutation led to an increase not only in the transcriptional activity of RXR but also in the activity of its heterodimeric complex with RAR or PPAR. SUSP1 co-localized with RXR in the nucleus, and removed SUMO-1 from RXR but not from androgen receptor or PPAR. Moreover, over-expression of SUSP1 caused an increase in the transcriptional activity of RXR, whereas shRNA-mediated knock down of endogenous SUSP1 led to a decrease in RXR activity. These results suggest that SUSP1 plays an important role in the control of the transcriptional activity of RXR, and thus in the RXR-mediated cellular processes.