Cohesin complexes maintain sister chromatid cohesion to ensure proper chromosome segregation during mitosis and meiosis. In plants, the exact components and functions of the cohesin complex remain poorly understood. Here, we positionally cloned the classic maize (Zea mays) mutant defective kernel 15 (dek15), revealing that it encodes a homolog of SISTER CHROMATID COHESION PROTEIN 4 (SCC4), a loader subunit of the cohesin ring. Developing dek15 kernels contained fewer cells than the wild type, but had a highly variable cell size. The dek15 mutation was found to disrupt the mitotic cell cycle and endoreduplication, resulting in a reduced endosperm and embryo lethality. The cells in the dek15 endosperm and embryo exhibited precocious sister chromatid separation and other chromosome segregation errors, including misaligned chromosomes, lagging chromosomes, and micronuclei, resulting in a high percentage of aneuploid cells. The loss of Dek15/Scc4 function upregulated the expression of genes involved in cell cycle progression and stress responses, and downregulated key genes involved in organic synthesis during maize endosperm development. Our yeast two-hybrid screen identified the chromatin remodeling proteins chromatin remodeling factor 4, chromatin remodeling complex subunit B (CHB)102, CHB105, and CHB106 as SCC4-interacting proteins, suggesting a possible mechanism by which the cohesin ring is loaded onto chromatin in plant cells. This study revealed biological functions for DEK15/SCC4 in mitotic chromosome segregation and kernel development in maize.