SummaryGrain size is an important factor determining yield in rice. Here, we identified a recessive mutant gene, grain length and awn 1 (gla1), which caused a significant increase in grain length and weight, and was associated with long awns. The gla1 mutation was mapped to a single‐nucleotide polymorphism in a gene encoding a cytoplasmically‐localized mitogen‐activated protein kinase (MAPK) phosphatase. Overexpression of GLA1 caused a decrease in grain length, and the GLA1 protein interacted with OsMAPK6. These results suggest that GLA1 may serve as a negative regulator of the OsMAPKK4‐OsMAPK6 cascade, controlling grain size via the dephosphorylation of OsMAPK6.