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The development of floral organs is coordinated by an elaborate network of homeotic genes, and gibberellin (GA) signaling is involved in floral organ development; however, the underlying molecular mechanisms remain elusive. In the present study, we found that MOS4-ASSOCIATED COMPLEX 5A (MAC5A), which is a protein containing an RNA-binding motif, was involved in the development of sepals, petals, and stamens; either the loss or gain of <i>MAC5A</i> function resulted in stamen malformation and a reduced seed set. The exogenous application of GA considerably exacerbated the defects in <i>mac5a</i> null mutants, including fewer stamens and male sterility. <i>MAC5A</i> was predominantly expressed in pollen grains and stamens, and overexpression of <i>MAC5A</i> affected the expression of homeotic genes such as <i>APETALA1</i> (<i>AP1</i>), <i>AP2</i>, and <i>AGAMOUS</i> (<i>AG</i>). MAC5A may interact with RABBIT EARS (RBE), a repressor of <i>AG</i> expression in <i>Arabidopsis</i> flowers. The petal defect in <i>rbe</i> null mutants was at least partly rescued in <i>mac5a rbe</i> double mutants. These findings suggest that <i>MAC5A</i> is a novel factor that is required for the normal development of stamens and depends on the GA signaling pathway.