The study investigates the role of OsSWEET1b, a hexose transporter protein localized in the plasma membrane of rice leaves, in sugar metabolism and leaf senescence. OsSWEET1b is highly expressed in leaves and has glucose and galactose transporter activity in yeast. The *ossweet1b* mutant plants exhibit reduced sugar content (sucrose, glucose, fructose, starch, and galactose) and increased carbon starvation-related gene expression, leading to carbon starvation in the leaves during the filling stage. These mutants also show decreased chlorophyll content, reduced antioxidant enzyme activity, and increased ROS accumulation, resulting in premature leaf senescence. The *ossweet1b* mutants display altered leaf senescence-related gene expression and reduced abundance of photosynthesis-related proteins. RNA-seq analysis revealed that loss of *OsSWEET1b* affects starch and sucrose metabolism and carbon fixation in photosynthetic organelles. The study concludes that OsSWEET1b plays a crucial role in regulating rice leaf carbohydrate metabolism and maintaining rice yield.The study investigates the role of OsSWEET1b, a hexose transporter protein localized in the plasma membrane of rice leaves, in sugar metabolism and leaf senescence. OsSWEET1b is highly expressed in leaves and has glucose and galactose transporter activity in yeast. The *ossweet1b* mutant plants exhibit reduced sugar content (sucrose, glucose, fructose, starch, and galactose) and increased carbon starvation-related gene expression, leading to carbon starvation in the leaves during the filling stage. These mutants also show decreased chlorophyll content, reduced antioxidant enzyme activity, and increased ROS accumulation, resulting in premature leaf senescence. The *ossweet1b* mutants display altered leaf senescence-related gene expression and reduced abundance of photosynthesis-related proteins. RNA-seq analysis revealed that loss of *OsSWEET1b* affects starch and sucrose metabolism and carbon fixation in photosynthetic organelles. The study concludes that OsSWEET1b plays a crucial role in regulating rice leaf carbohydrate metabolism and maintaining rice yield.