Seed size/weight is a critical factor determining crop yield; however, a limited number of genes regulating this trait have been characterized in soybean. In this study, we identified a Glycine max CELL DIVISION CYCLE 7 (GmCDC7) and revealed its essential roles in seed development. The putative GmCDC7 was highly conserved in both sequences and structure across various species. GmCDC7 transcripts were detectable in multiple tissues, with peak expression occurring during early seed development, while the GmCDC7 proteins were predominantly localized within the nucleus. CRISPR/Cas9-mediated knockout of GmCDC7 led to a significant increase in seed size and 100-seed weight, while overexpression of this gene resulted in a reduction in both seed size and weight. Further cytological analysis demonstrated that GmCDC7 promoted cell expansion and inhibited cell proliferation in seeds. Notably, the gene-edited gmcdc7 mutants showed a substantial increase in protein content alongside a reduction in oil content in seeds. Correspondingly, transcriptomic analyses revealed that GmCDC7 may significantly influence multifaceted regulatory pathways related to cell cycle-related activities, storage protein accumulation, and lipid transport and metabolism during seed development. These findings suggest that GmCDC7 plays pivotal roles in modulating seed size/weight and quality, offering new gene resources and insights into biotechnological strategies for soybean breeding.