Introducing genomic segments from related species into crops is an effective approach to enrich genetic diversity and create novel germplasm. African cultivated rice (Oryza glaberrima), one of the two cultivated rice species, represents a valuable genetic resource for improving Asian cultivated rice (O. sativa). Grain size/weight is among the most important components of grain yield in rice. In this study, we isolated a chromosome segment introgression line harboring a single segment derived from O. glaberrima. Compared to the recurrent parent, this line displayed a significant increase in grain size, whereas other agronomic traits were not significantly affected. Using map-based cloning, we identified OgGS3.2 from this fragment as an ortholog of OsSLR1. A single nucleotide substitution in OgGS3.2 was identified as the causal mutation underlying the increased seed size phenotype. Haplotype analysis revealed that the vast majority of African rice germplasm carries the G haplotype at the GS3.2 locus. This study reveals the molecular mechanism by which OgGS3.2 regulates grain size and offers strategic genetic resources for yield improvement in cultivated rice.