Grain size is a critical yield determinant and a complex quantitative trait in rice. The major quantitative trait locus GS2, which encodes the transcriptional regulator OsGRF4, has been characterized as a key controller of grain size and grain weight in rice. Building upon existing germplasm resources, exploring the upstream and downstream genes of known grain size regulators is an effective approach to gradually refine and expand the molecular regulatory network underlying grain size. Here, we report that a suppressor of the gain-of-function allele GS2^AA, SUG2, which encodes an importin β nuclear transport protein. Phenotypic characterization reveals that the sug2 GS2^AA mutant exhibits small grains by inhibiting cell expansion in the spikelet hull. Protein interaction analyses demonstrate that SUG2 physically interacts with GS2 both in vivo and in vitro, and the SUG2^A mutation reduces SUG2 expression levels and inhibits GS2 transcriptional activation activity, resulting in decreased GS2 expression levels and GS2 protein abundance in sug2 GS2^AA. Genetic analyses indicate that SUG2 and GS2 may be partially involved in a common pathway regulating grain size and weight in rice. These findings elucidate the grain size regulatory relationship between SUG2 and GS2 and provide novel insights into the precision breeding of yield optimization in rice.