Abstract: Following double fertilization, plant endosperm nuclei undergo syncytial divisions, followed by synchronous cellularization. Cellularization is a key event during endosperm development, but our understanding of its regulation is limited to Arabidopsis. In this study we show that OsbZIP76 regulates cellularization in rice (Oryza sativa). Activation of OsbZIP76 coincided with the initiation of cellularization, and its knockdown or knockout mutants exhibited precocious cellularization. Genes involved in endosperm development or starch biosynthesis were prematurely activated in the osbzip76 caryopsis. As a putative transcription factor, OsbZIP76 alone lacked transcriptional activation activity; however, it interacted with the nuclear factor Y (NF‐Y) family transcription factors OsNF‐YB9 and OsNF‐YB1 in yeast and in planta. OsbZIP76 and OsNF‐YB9 were predominantly expressed in the endosperm and the proteins colocalized. Seeds of osnf‐yb1 and osbzip76 mutants showed reduced size and reduced apparent amylose content. The parent‐of‐origin‐dependent expression of OsbZIP76 is variable in different rice accessions. In summary, OsbZIP76 is an endosperm‐expressed imprinted gene that regulates endosperm development in rice.