Functions as an E3-type small ubiquitin-like modifier (SUMO) ligase, stabilizing the interaction between UBE2I and the substrate, and as a SUMO-tethering factor. Plays a crucial role as a transcriptional coregulation in various cellular pathways, including the STAT pathway and the steroid hormone signaling pathway. Involved in regulating STAT3 signaling via inhibiting STAT3 DNA-binding and suppressing cell growth. Enhances the sumoylation of MTA1 and may participate in its paralog-selective sumoylation (PubMed:21965678, PubMed:9388184). Sumoylates CCAR2 which promotes its interaction with SIRT1 (PubMed:25406032). Diminishes the sumoylation of ZFHX3 by preventing the colocalization of ZFHX3 with SUMO1 in the nucleus (PubMed:24651376). Protein modification; protein sumoylation. Monomer (By similarity). Binds SUMO1 and UBE2I. Interacts with BCL11A, HMGA2, IRF1, MITF and NCOA2. Interacts with STAT5; the interaction occurs on stimulation by PRL. Interacts with GFI1; the interaction relieves the inhibitory effect of PIAS3 on STAT3-mediated transcriptional activity (By similarity). Interacts with AR, PLAG1 and ZFHX3. Interacts with STAT3; the interaction occurs on stimulation by IL6, CNTF or OSM and inhibits the DNA binding activity of STAT3. Interacts with MTA1. Interacts with CCAR2 (via N-terminus). Interacts with TRIM8 (PubMed:20516148). Interacts with PRDM1/Blimp-1 (PubMed:28842558). Colocalizes with MITF in the nucleus. Colocalizes with GFI1 in nuclear dots. Colocalizes with SUMO1 in nuclear granules. Widely expressed. By dihydrotestosterone (DHT) in prostate cancer cells. The PINIT domain of PIAS3 is required for STAT3-PIAS3 interaction and for translocation to the nucleus. The LXXLL motif is a transcriptional coregulator signature. Sumoylated. Belongs to the PIAS family.