Summary Sporulation in Bacillus subtilis is governed by a cascade of alternative RNA polymerase sigma factors. We previously identified a small protein Fin that is produced under the control of the sporulation sigma factor σF to create a negative feedback loop that inhibits σF‐directed gene transcription. Cells deleted for fin are defective for spore formation and exhibit increased levels of σF‐directed gene transcription. Based on pull‐down experiments, chemical crosslinking, bacterial two‐hybrid experiments and nuclear magnetic resonance chemical shift analysis, we now report that Fin binds to RNA polymerase and specifically to the coiled‐coil region of the β′ subunit. The coiled‐coil is a docking site for sigma factors on RNA polymerase, and evidence is presented that the binding of Fin and σF to RNA polymerase is mutually exclusive. We propose that Fin functions by a mechanism distinct from that of classic sigma factor antagonists (anti‐σ factors), which bind directly to a target sigma factor to prevent its association with RNA polymerase, and instead functions to inhibit σF by competing for binding to the β′ coiled‐coil. During the developmental process of sporulation in Bacillus subtilis a cascade of alternative sigma factors associate with RNA polymerase to direct gene expression. We report that Fin, a novel RNAP‐binding protein produced during sporulation, inhibits the function of the sporulation sigma factor σF by competing with the binding of region 2 (Rg2) of σF to the coiled‐coil region of the β′ subunit of RNA polymerase (β′ CC).