Inhibition of cell division is critical for viability under DNA-damaging conditions. DNA damage induces the SOS response that in bacteria inhibits cell division while repairs are being made. In coccoids， such as the human pathogen， Staphylococcus aureus， this process remains poorly studied. Here， we identify SosA as the staphylococcal SOS-induced cell-division inhibitor. Overproduction of SosA inhibits cell division while sosA inactivation sensitizes cells to genotoxic stress. SosA is a small， predicted membrane protein with an extracellular C-terminal domain in which point mutation of residues that are conserved in staphylococci and major truncations abolished the inhibitory activity. In contrast， a minor truncation led to SosA accumulation and a strong cell division-inhibitory activity， phenotypically similar to expression of wildtype SosA in a CtpA membrane protease mutant. This suggests that the extracellular C-terminus of SosA is required both for cell division inhibition and for turnover of the protein. Microscopy analysis revealed that SosA halts cell division and synchronizes the cell population at a point where division proteins such as FtsZ and EzrA are localized at mid-cell， and the septum formation is initiated but unable to progress to closure. Thus， our findings show that SosA is central in cell division regulation in staphylococci. This article is protected by copyright. All rights reserved.