OsSWEET1b is a hexose transporter localized in the plasma membrane of rice cells and forms homodimers. Knockout of OsSWEET1b reduces leaf sugar content and accelerates leaf senescence. The SWEET gene family in rice includes 21 members, but the function of OsSWEET1b is not well understood. Expression analysis showed that OsSWEET1b is highly expressed in leaves. Transport experiments in yeast confirmed that OsSWEET1b has glucose and galactose transporter activity. Subcellular localization and BiFC analysis showed that OsSWEET1b is localized to the plasma membrane and forms homodimers. Functional analysis of the ossweet1b mutant showed reduced sucrose, glucose, fructose, starch, and galactose contents, increased carbon starvation-related gene expression, decreased chlorophyll content and antioxidant enzyme activity, increased ROS accumulation, and premature senescence. Leaf senescence-related gene expression was increased, and photosynthesis-related protein abundance was decreased. RNA-seq analysis showed that OsSWEET1b loss affects starch, sucrose metabolism, and carbon fixation in photosynthetic organelles. The loss of OsSWEET1b function leads to sugar starvation, reduced photosynthesis, and leaf senescence, which reduces rice yield. OsSWEET1b plays a key role in rice leaf carbohydrate metabolism and leaf senescence. SWEET proteins are involved in sugar transport in plants and are divided into four clades. In Arabidopsis, clade I and II members are mainly responsible for hexose transport, clade III for sucrose, and clade IV for fructose. In crops, some SWEET proteins are involved in assimilate transport from source to sink, which is crucial for crop yields. In rice, OsSWEETs include 21 members, with OsSWEET4, OsSWEET5, OsSWEET11, OsSWEET14, and OsSWEET15 involved in grain filling. OsSWEET1b belongs to a different subfamily. This study characterized the function of OsSWEET1b in rice, showing that it is a glucose and galactose transporter, predominantly expressed in leaves, localized to the plasma membrane, and involved in regulating rice leaf carbohydrate metabolism and yield.OsSWEET1b is a hexose transporter localized in the plasma membrane of rice cells and forms homodimers. Knockout of OsSWEET1b reduces leaf sugar content and accelerates leaf senescence. The SWEET gene family in rice includes 21 members, but the function of OsSWEET1b is not well understood. Expression analysis showed that OsSWEET1b is highly expressed in leaves. Transport experiments in yeast confirmed that OsSWEET1b has glucose and galactose transporter activity. Subcellular localization and BiFC analysis showed that OsSWEET1b is localized to the plasma membrane and forms homodimers. Functional analysis of the ossweet1b mutant showed reduced sucrose, glucose, fructose, starch, and galactose contents, increased carbon starvation-related gene expression, decreased chlorophyll content and antioxidant enzyme activity, increased ROS accumulation, and premature senescence. Leaf senescence-related gene expression was increased, and photosynthesis-related protein abundance was decreased. RNA-seq analysis showed that OsSWEET1b loss affects starch, sucrose metabolism, and carbon fixation in photosynthetic organelles. The loss of OsSWEET1b function leads to sugar starvation, reduced photosynthesis, and leaf senescence, which reduces rice yield. OsSWEET1b plays a key role in rice leaf carbohydrate metabolism and leaf senescence. SWEET proteins are involved in sugar transport in plants and are divided into four clades. In Arabidopsis, clade I and II members are mainly responsible for hexose transport, clade III for sucrose, and clade IV for fructose. In crops, some SWEET proteins are involved in assimilate transport from source to sink, which is crucial for crop yields. In rice, OsSWEETs include 21 members, with OsSWEET4, OsSWEET5, OsSWEET11, OsSWEET14, and OsSWEET15 involved in grain filling. OsSWEET1b belongs to a different subfamily. This study characterized the function of OsSWEET1b in rice, showing that it is a glucose and galactose transporter, predominantly expressed in leaves, localized to the plasma membrane, and involved in regulating rice leaf carbohydrate metabolism and yield.