PML nuclear bodies (NBs) are nuclear structures that have been implicated in processes such as transcriptional regulation, genome stability, response to viral infection, apoptosis, and tumor suppression. PML has been found to be essential for the formation of the NBs, as these structures do not form in Pml null cells, although PML add back fully rescues their formation. However, the basis for such a structural role of PML is unknown. We demonstrate that PML contains a SUMO binding motif that is independent of its SUMOylation sites and is surprisingly necessary for PML-NB formation. We demonstrate that the PML RING domain is critical for PML SUMOylation and PML-NB formation. We propose a model for PML-NB formation whereby PML SUMOylation and noncovalent binding of PML to SUMOylated PML through the SUMO binding motif constitutes the nucleation event for subsequent recruitment of SUMOylated proteins and/or proteins containing SUMO binding motifs to the PML NBs.