Cleistogamy, or self-fertilization of a closed flower, can limit unintended gene flow and may contribute to varietal purity in rice (Oryza sativa), with potential value for transgene containment. We previously identified the natural cleistogamous mutant lodiculeless spikelet (ld). Here, using map-based cloning, we show that ld carries a 4.6-kb deletion encompassing the entire MICRORNA806a (MIR806a) precursor and the upstream region of the MIR172b locus. CRISPR/Cas9-mediated editing of the MIR806a or MIR172b locus demonstrated that the loss of accumulation of miR172b, but not miR806a, is responsible for the cleistogamy phenotype of ld plants. Small RNA sequencing confirmed that miR172b is nearly absent in ld mutants. Among the five APETALA 2 (AP2)-like genes harboring miR172b target sites, only SUPERNUMERARY BRACT (SNB) transcripts accumulated to significantly higher levels in young ld panicles than the wild type. Prime editing of the miR172-binding site in SNB generated a miR172-resistant SNB transcript isoform that reproduced the vestigial lodicule phenotype, indicating that the derepression of SNB transcript accumulation is sufficient to alter lodicule development. Histological analysis of rice harboring the GUS reporter gene driven by the MIR172b or SNB promoter revealed a strong overlap between the MIR172b and SNB expression domains in developing lodicules, supporting their regulatory relationship. Together, these results identify the miR172b-mediated repression of SNB transcript abundance as an important regulatory module for lodicule development and cleistogamy in rice. The agronomically neutral ld alleles represent valuable genetic resources for developing cleistogamous rice cultivars.