Kernel weight is a critical agronomic trait in maize production. Many genes are related to kernel weight but only a few of them have been applied to maize breeding and cultivation. Here, we identify a novel function of maize mitogen-activated protein kinase 6 (ZmMPK6) in the regulation of maize kernel weight. Kernel weight was reduced in zmmpk6 mutants and increased in ZmMPK6-overexpressing lines. In addition, starch granules, starch content, protein content, and grain-filling characteristics were also affected by the ZmMPK6 expression level. ZmMPK6 is mainly localized in the nucleus and cytoplasm, widely distributed across various tissues, and is expressed during kernel development, which is consistent with its role in kernel weight. Thus, these results provide new insights into the role of ZmMPK6, a mitogen-activated protein kinase, in maize kernel weight, and could be applied to further molecular breeding for kernel quality and yield in maize.