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Efficient ultraviolet (UV) electroluminescent materials remain a great challenge, since short peak wavelength <400 nm and narrow full width at half maximum (FWHM) <50 nm are simultaneously required. In this sense, multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters featuring narrow‐band emissions hold the promise for UV applications. Herein, a novel MR‐TADF skeleton featuring carbazole‐phosphine oxide (P=O) fused aromatics is developed to construct the first two UV MR emitters named CzP2PO and tBCzP2PO. In addition to synergistic resonance effects of P=O and N atom, sp3‐hybrid P atom renders the curved polycyclic planes of CzP2PO and tBCzP2PO, giving rise to their narrowband UV emissions with peak wavelengths <390 nm and FWHM<35 nm. Besides configuration quasi‐planarization for radiation enhancement and quenching suppression, P=O moiety further enhances singlet‐triplet coupling to facilitate reverse intersystem crossing, resulting in the state‐of‐the‐art photoluminescence quantum yield of 62 % in tBCzP2PO doped films. As consequence, tBCzP2PO endowed its UV organic light‐emitting diodes with the peak at 382 nm and FWHM of 32 nm, and especially the record‐high external quantum efficiency (EQE) of 15.1 % among all kinds of UV devices. Our results demonstrate great potential of P=O based MR emitters in practical applications including optoelectronics, biology and medicine science. [ABSTRACT FROM AUTHOR]