Grain size and shape are two crucial traits that influence grain yield and grain appearance in rice. Although several factors that affect grain size have been described in rice, the molecular mechanisms underlying grain size and shape determination are still elusive. In this study, we report that the WIDE AND THICK GRAIN 1 (WTG1) functions as an important factor that determines grain size and shape in rice. The wtg1-1 mutant exhibits wide, thick, short and heavy grains and also shows the increased grain number per panicle. WTG1 determines grain size and shape mainly by influencing cell expansion. WTG1 encodes an otubain-like protease, which shares similarity with human OTUB1. Biochemical analyses indicate that WTG1 is a functional deubiquitinating enzyme, and the mutant protein (wtg1-1) loses the activity of deubiquitinating enzyme. WTG1 is expressed in developing grains and panicles, and the GFP-WTG1 fusion protein is present in the nucleus and the cytoplasm. Overexpression of WTG1 results in narrow, thin and long grains due to narrow and long cells, further supporting the role of WTG1 in determining grain size and shape. Thus, our findings identify the otuba