In maize (Zea mays), sex determination occurs through abortion of female carpels in the tassel and arrest of male stamens in the ear. The Tasselseed6 (Ts6) and tasselseed4 (ts4) mutations permit carpel development in the tassel while increasing meristem branching, showing that sex determination and acquisition of meristem fate share a common pathway. We show that ts4 encodes a mir172 microRNA that targets APETALA2 floral homeotic transcription factors. Three lines of evidence suggest that indeterminate spikelet1 (ids1), an APETALA2 gene required for spikelet meristem determinacy, is a key target of ts4. First, loss of ids1 suppresses the ts4 sex determination and branching defects. Second, Ts6 mutants phenocopy ts4 and possess mutations in the microRNA binding site of ids1. Finally, IDS1 protein is expressed more broadly in ts4 mutants compared to wild type. Our results demonstrate that sexual identity in maize is acquired by limiting floral growth through negative regulation of the floral homeotic pathway.