Narrow-band deep-blue phosphorescent emitters with high efficiency remain challenging for organic light-emitting diodes (OLEDs). Herein, a cyclometalated platinum(II) complex, Pt5, was designed and synthesized with a rigid molecular structure to suppress vibronic coupling. Pt5 exhibits deep-blue photoluminescence at 451 nm in dichloromethane with an ultranarrow full width at half maximum (FWHM) of 23 nm. Low-temperature photoluminescence reveals a high triplet energy of 2.78 eV. The HOMO and LUMO energy levels are −5.28 and −2.30 eV, respectively, with an optical bandgap of 2.98 eV, and a decomposition temperature of 315 °C. Solution-processed OLEDs based on Pt5 achieve a maximum external quantum efficiency of 23.9% and a maximum luminance of 8820 cd m⁻², showing stable deep-blue electroluminescence at 456 nm with CIE coordinates of (0.142, 0.121).