Grain size is a key factor influencing grain weight in rice. In this study, a chromosome segment substitution line (CSSL9-17) was identified, that exhibits a significant reduction in both grain size and weight compared to its donor parent 93-11. Further investigation identified two quantitative trait loci (QTL) on chromosome 11, designated qGW11a and qGW11b, which contribute to 1000-grain weight with an additive effect. LOC_Os11g05690, encoding the amino acid permease OsCAT8, is the target gene of qGW11a. Overexpression of OsCAT8 resulted in decreased grain weight, while OsCAT8 knockout mutants exhibited increased grain weight. The 287-bp located within the OsCAT8 promoter region of 93-11 negatively regulates its activity, which is subsequently correlated with an increase in grain size and weight. These results suggest that OsCAT8 functions as a negative regulator of grain size and grain weight in rice.