As a multifunctional protein, Yin Yang 1 (YY1) has been demonstrated to regulate both gene expression and protein posttranslational modifications. However, gaps still exist in our knowledge of how YY1 can be modified and what the consequences of its modifications are. Here we report that YY1 protein can be sumoylated both in vivo and in vitro. We have identified lysine 288 as the major sumoylation site of YY1. We also discovered that PIASy, a SUMO E3 ligase, is a novel YY1-interacting protein and can stimulate the sumoylation of YY1 both in vitro and in vivo. Importantly, the effects of PIASy mutants on in vivo YY1 sumoylation correlate with the YY1-PIASy interaction but do not depend on the RING finger domain of PIASy. This regulation is unique to YY1 sumoylation because PIASy-mediated p53 sumoylation still relies on the integrity of PIASy, which is also true of all of the previously identified substrates of PIASy. In addition, PIASy colocalizes with YY1 in the nucleus, stabilizes YY1 in vivo, and differentially regulates YY1 transcriptional activity on different target promoters. This study demonstrates that YY1 is a target of SUMOs and reveals a novel feature of a SUMO E3 ligase in the PIAS family that selectively stimulates protein sumoylation independent of the RING finger domain.