HD-ZIP proteins (homeodomain–leucine zipper proteins) are a large family of plant-specific transcription factors that play crucial roles in regulating various physiological and developmental processes, including growth, differentiation, response to environmental stress, and reproductive development. Seed size is the main limiting factor affecting the yield of maize (Zea mays). However, the genome-wide identification and characterization of this family in maize and its biological functions in seed size have not been reported. Here, 61 ZmHD-ZIP genes were identified in the genome. Phylogenetic analysis of these ZmHD-ZIP genes revealed that they are clustered into four subfamilies: HD-ZIP I, HD-ZIP II, HD-ZIP III, and HD-ZIP IV. Domain analysis revealed that the distribution of these domains aligns perfectly with the subfamily classification criteria, with members of the same subfamily sharing similar domains. Cis-acting element analysis showed that the ZmHD-ZIP family genes are involved in the regulation of plant reproductive development. In addition, ZmHD-ZIP23-overexpressing Arabidopsis and maize had larger seed with increased grain length and heavier grain weight owing to bigger embryo and endosperm area. These findings could serve as a basis for future studies on the roles of ZmHD-ZIP genes in improving maize yield.