Yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic (YAM) and yttrium aluminium perovskite (YAP). YAG is commonly used as a host material in various solid-state lasers. Rare earth elements such as neodymium and erbium can be doped into YAG as active laser ions, yielding Nd:YAG and Er:YAG lasers, respectively. Cerium-doped YAG (YAG:Ce) is used as a phosphor in cathode ray tubes and white light-emitting diodes, and as a scintillator.

YAG for a period was used in jewelry as a diamond and other gemstone simulant. Colored variants and their doping elements include: green (chromium), blue (cobalt), red (manganese), yellow (titanium), purple (neodymium), pink, and orange. As faceted gems they are valued (as synthetics) for their clarity, durability, high refractive index and dispersion. The critical angle of YAG is 33 degrees. YAG cuts similar to natural garnet, with polishing being performed with alumina or diamond (50,000 or 100,000) on common polishing laps. YAG has low heat sensitivity.