<p>In this study, YAG:Ce ceramics doped with ruthenium atoms were synthesized for the first time. The effects of ruthenium on the luminescence and colorimetric characteristics of YAG:Ce ceramics are discussed, with variations in the type of sintering additives. The luminescence spectra, chromaticity coordinates, color rendering index, correlated color temperature, luminous flux, and luminous efficiency were studied. For all ruthenium-containing compositions, the color rendering index increases, the color temperature rises, and the chromaticity coordinates shift towards those of white light. An increase in the color rendering index by several percentage points (approximately 8–9.2%) was observed for all ruthenium-containing compositions compared to ruthenium-free ones. This observed increase in the color rendering index of YAG:Ce,Ru ceramic samples is due to the increased proportion of the blue component in the emission spectra. It was shown that incorporating ruthenium atoms into the garnet structure leads to a decrease in luminescence intensity. The decrease in luminescence intensity is attributed to non-radiative energy transfer from Ce³⁺ to Ru ions, as supported by absorption and temperature-dependent measurements. These results can be used to improve the composition of luminescent ceramics.</p>