Chinese cabbage (Brassica rapa subsp. pekinensis) is an important leafy vegetable in the Brassica genus of the Brassicaceae family. The size of its edible leaves is an essential trait that determines its economic and nutritional values. However, the current understanding of leaf development in Chinese cabbage remains limited. Here, through forward genetic analysis of the mutant mini24 with defective leaf and root development, we identified the BrRRG gene, which regulates cell division in Chinese cabbage by map-based cloning. We demonstrated that BrRRG impacts leaf size by regulating the expression of E2Fa transcription factors and cell cycle-related genes in Chinese cabbage. Furthermore, BrRRG was found to modulate Chinese cabbage’s response to auxin (indole-3-acetic acid, IAA) and cytokinins hormones, revealing a distinct regulatory mechanism by which BrRRG coordinates the development of underground roots and aboveground leaves. Thus, these results indicate that mutation in BrRRG impairs the growth and development of Chinese cabbage.