Endoreduplication is prevalent during plant growth and development, and is often correlated with large cell and organ size. Despite its prevalence, the transcriptional regulatory mechanisms underlying the transition from mitotic cell division to endoreduplication remain elusive. Here, we characterize ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR 4 (ERF4) as a positive regulator of endoreduplication through its function as a transcriptional repressor. ERF4 was specifically expressed in mature tissues in which the cells were undergoing expansion, but was rarely expressed in young organs. Plants overexpressing ERF4 exhibited much larger