Kernel weight is an important yield component in maize that was selected during domestication. Many kernel weight genes have been identified through mutant analysis, and are mostly involved in the biogenesis and functional maintenance of organelles or other fundamental cellular activities. However, only a limited number of loci underlying quantitative variation in kernel weight have been cloned. Here we characterize a maize kernel weight QTL, qKW9 and find that it encodes a DYW motif pentatricopeptide repeat protein involved in C-to-U editing of NdhB, a subunit of the chloroplast NADH dehydrogenase-like complex. In a null qKW9 background, C-to-U editing of NdhB was abolished, and photosynthesis was reduced, suggesting that qKW9 regulates kernel weight by controling the maternal source of photosynthate for grain filling. Characterization of qKW9 highlights the importance of optimizing photosynthesis on maize grain yield production.