Leaf biomass accumulation is a critical determinant of photosynthetic capacity and crop productivity. In Arabidopsis thaliana, multiple hormonal and environmental pathways, including brassinosteroid (BR), auxin and light signaling, as well as functional proteins such as TNY (TINY), SAUR21 (SMALL AUXIN UP-RNA21) and HY5 (ELONGATED HYPOCOTYL 5), play important roles in regulating leaf growth. However, the precise regulatory mechanisms integrating these factors during leaf biomass accumulation remain incompletely understood. Herein, we showed that NF-YC3/4/9, members of the NUCLEAR FACTOR Y subunit C family, were required for normal leaf cell expansion. Loss-of-function mutation of NF-YC3/4/9 (nf-ycT) resulted in significantly smaller true leaves with reduced leaf cell expansion. NF-YC9 directly regulated the expression of HY5, HYH, and SAUR21 and indirectly regulated the expression of TNY. These results help reveal the function of NF-YCs in leaf growth and provide insights into the regulation of hormonal and transcriptional networks controlling leaf biomass accumulation in A. thaliana.