Grain quality is a critical component of high-yielding varieties to ensure acceptance by an ever-increasing population and living standards. During the past years, several photorespiration bypasses have been introduced into C3 plants, among which our GOC and GCGT bypasses exhibit increased photosynthesis and yield in rice. However, to the best of our knowledge, there are still no reports referring to effects of the bypasses on grain quality. Thus, the objective of this study is to determine the effect of GOC and GCGT bypasses on grain quality, and the mechanism of how photorespiratory bypasses affect grain quality was also investigated. Compared with the WT of Zhonghua 11, GOC4 and GCGT20 plants had higher nutritional quality and cooking quality as grain protein content was significantly increased by 11.27% and 14.97%, and alkali spreading value was significantly increased by 7.6% and 4.63%, respectively, whereas appearance quality appears to be negatively affected since the chalky rice rate was increased by 32.6% and 68%, respectively. Analyses also demonstrated that the changes in grain quality may result from the increased total nitrogen and constrained carbohydrate transport in the transgenic plants. Altogether, the results not only suggest that the increased photosynthesis and yield by introducing the photorespiratory bypasses can significantly affect grain quality parameters for rice, either positively or negatively, but also imply that the coordination of source–sink transport may play important roles in grain quality formation for high-yielding crops via increased photosynthetic efficiency.