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SUMMARY: The CUP‐SHAPED COTYLEDON (CUC) genes (CUC1, CUC2 and CUC3) regulate organ boundary formation in Arabidopsis. However, the functions of their homologous genes in rice (Oryza sativa) are still unknown. Here, we have identified an orthologous gene of CUC1 and CUC2 in rice, named OsNAM. Subcellular localization and yeast two‐hybrid assay results have suggested that OsNAM encodes a conserved nuclear NAC (NAM/ATAF1/CUC2) protein with a transcriptional activator. The null mutant osnam‐1 presented a fused leaf structure, small panicles, reduced branches and aberrant floral organ identities when compared with those of the wild type. Beta‐glucuronidase staining and GFP reporter lines indicated that OsNAM was expressed in young tissues and that its boundary enrichment expression was regulated by OsmiR164. Loss‐of‐function mutants for OsCUC3 resulted in no obvious defects throughout rice development. The osnam oscuc3 double mutant, however, resulted in severe leaf fusion of the first two leaves, while the osnam single mutant showed a similar phenotype from the seventh leaf. These results indicated that OsNAM and OsCUC3 act redundantly for boundary specification during post‐embryonic development. Overall, we describe the biological functions of OsNAM and OsCUC3 in rice development and the expression characteristics of OsNAM. This work reveals the important role of CUC genes in rice. [ABSTRACT FROM AUTHOR]