SIN3 is a large protein providing a platform for the assembly of multiple proteins, and SIN3 acting as a repressor negatively regulates genes involved in diverse cellular functions. SIN3 has been widely studied in yeasts and animals, but its function remains largely uncharacterized in plants. In this study, we identified and characterized three SIN3 homologous genes, SlSIN3a, SlSIN3b and SlSIN3c in tomato. The transcripts of SlSIN3b and SlSIN3c were almost undetectable, while SlSIN3a was highly expressed in all examined tissues (root, hypocotyl, cotyledon, leaf and fruit). We generated knockout mutagenesis of SlSIN3a using CRISPR/Cas9 technique. The slsin3a mutant plants exhibited abaxialized leaves compared with the wild-type. RNA-seq analysis indicated that the expressions of key leaf polarity regulators (SlTCP3, and SlHZ13), auxin signaling gene (SlIAA1), as well as cell expansin gene (SlEXPB2) and cyclin genes, were significantly changed in slsin3a mutant plants as compared to wild-type. We propose that SlSIN3a may be involved in leaf polarity through regulating auxin signaling, TCP, HD-ZIP and LBD genes, therefore affecting a serious of expansin and cyclin genes. Taken together, the results reveal a new role of SIN3 in leaf polarity and provide a possible action mechanism of SlSIN3a in leaf development.