It is well-known that doping YAG with Si dramatically affects densification and grain growth, and as a result Si is commonly added to YAG as a sintering additive to achieve full density and transparency. In recent studies, the influence of Si was explored, but segregation of Si to grain boundaries in YAG was not detected. The present article contradicts previous findings by revealing an excess of Si at grain boundaries in YAG. The findings were corroborated using atom probe tomography and energy-dispersive spectroscopy in a scanning transmission electron microscope. To the best of the author's knowledge, this is the first time Si segregation was discovered in SiO2-doped YAG, and the first time a dopant concentration profile at grain boundaries in a ceramic material was characterized by atom probe tomography. Finally, a change is proposed for the method to calculate grain-boundary mobility in the presence of a solute (Lucke and Detert), showing a good correspondence to the experimental results. These results can change the view on solute distribution in ceramic grain boundaries from theoretical and characterization aspects.