Androgen receptor (AR)-interacting protein 3 (ARIP3/PIASxa) is a coregulator capable of modulating transcriptional activity of various steroid receptors. We have characterized functional regions of ARIP3 and studied its interaction with the glucocorticoid receptor (GR)-interacting protein 1 (GRIP1). We find that the potential zinc-binding domain is critical for ARIP3 to function as a coactivator; the deletion of amino acids 347–418 or the mutation of the conserved cysteines 385 and 388 to serines converts ARIP3 to a transcriptional repressor from AR-dependent minimal promoters and abolishes its ability to activate GR. By contrast, mutations in the two LXXLL motifs of ARIP3 have relatively minor effects on its ability to regulate AR or GR function. ARIP3 is able to interact with different regions of GRIP1, but the strongest interaction is detected with the C-terminal region (amino acids 1122–1462) of GRIP1. The interaction of ARIP3 with the latter GRIP1 domain or full-length GRIP1 and the ability of ARIP3 to cooperate with GRIP1 in the regulation of AR- or GR-dependent transcription are dependent on the ARIP3 zinc-binding region. We also find a strong synergism between GRIP1 and two other PIAS family members, Miz1 and PIAS1. Taken together, our results suggest that PIAS proteins and GRIP1 interact functionally in transcriptional regulation.