The SUMO pathway mainly functions to repress innate immunity in myeloid cells. Inactivating sumoylation triggers a strong, noncanonical, type I interferon (IFN1) response, amplified and coupled with inflammation upon stimulation. These findings transposed to pre-clinical models with the demonstration that sumoylation inhibitors activate antitumor immunity in an IFN1-dependent manner. Yet, how sumoylation represses immune signaling remains largely unknown. Here, we identified MORC3, a negative regulator of IFNB1, as the top SUMO2/3 substrate in myeloid cells. We show that, in monocytes, SUMO functions to repress basal IFNB1 in cis through a single long tandem repeat regulated by MORC3 [MORC3-regulated element (MRE)] that concentrates multiple motifs for the myeloid-enriched PU.1 factor. Inhibiting sumoylation induces a 3D genome reorganization nucleated from the MRE, which acquires both insulator and PU.1-activated enhancer activities, together with loss of H3.3 and H3K9me3 repressive marks and recruitment of PU.1. Paradoxically, MORC3, that interacts with PU.1, is massively recruited, yet unable to repress the MRE. Finally, we show that both sumoylation and MORC3 ATPase cycle are critical for MORC3 repressive activity. Our study thus uncovers an unconventional mechanism in which sumoylation, in concert with MORC3, orchestrates a metastable H3.3/H3K9me3 heterochromatin state on a multi-PU.1 binding platform to prevent an uncontrolled myeloid-specific immune response.