Kernel weight is a critical agronomic trait in maize production. Many genes are related to it, but only a few of them have been applied to maize breeding and cultivation. Here, we identify a novel function of maize mitogenactivated protein kinase 6 (ZmMPK6) in the regulation of maize kernel weight. The kernel weight reduced in zmmpk6 mutants, while enhanced in ZmMPK6 overexpression lines. In addition, starch granules, starch content, protein content, and grainfilling characteristics are also affected. ZmMAPK6 is mainly localized in the nucleus and cytoplasm, widely distributed across various tissues, and expresses during kernel development, which is consistent with its role in kernel weight. Thus, these results denote new insights into the role of ZmMAPK6, a MAPK, in maize kernel weight, and could be applied to further molecular breeding for kernel quality and yield in maize.