Abstract Large‐size ultrathin phosphor‐glass composites (PGCs), characterized by superior thermal dissipation properties, have demonstrated considerable potential for high‐power illumination and adjustable color emission. Here, a facile and versatile strategy is presented to fabricate highly efficient ultrathin PGCs with dimensions reaching 1044 × 45 × 0.1 mm 3 , using an integrated tape‐casting and low‐temperature co‐firing approach. Phosphor particles are embedded into the glass matrix via a pressureless sintering process. The impact of the glass matrix on the luminescent properties of both yellow and red phosphors is systematically evaluated. These findings indicate that ultrathin PGCs embedded with YAG:Ce particles exhibit an exceptionally high photoluminescence quantum yield (PLQY) of 98.6%, while those incorporating CaAlSiN 3 :Eu 2 ⁺ phosphors achieve a PLQY of ≈93.4%. Additionally, comprehensive analyses are conducted on spectral blue shifts, luminescence lifetime variations, and temperature‐dependent photoluminescence characteristics over a broad temperature range. Light‐emitting diode (LED) devices fabricated using these ultrathin PGCs show superior luminous performance, notably exhibiting a high electrical power threshold prior to luminescence saturation. Furthermore, it is successfully demonstrated tunable color emission in these ultrathin PGCs by varying the weight ratios of different phosphors. These remarkable ultrathin PGCs represent a significant advancement, providing promising and highly efficient solutions for next‐generation high‐power commercial LED illumination applications.