A B–N covalent bond-involved π-extension strategy was developed to construct multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters, enabling high-performance narrowband electroluminescence. The strategy involves post-functionalization of MR frameworks to generate high-order B/N-based motifs, leading to extended π-systems that enhance molecular rigidity and promote reverse intersystem crossing (RISC). This results in ultra-narrowband emitters with improved photophysical properties, such as high photoluminescence quantum yields and reduced emission linewidths. The developed emitters, DABNA-3B and BCzBN-3B, achieved external quantum efficiencies (EQE) of up to 42.6% in narrowband OLEDs, with significantly reduced efficiency roll-off at high brightness. These emitters demonstrated exceptional performance, including narrow emission bands (as narrow as 8 nm in n-hexane), high Φ_PL values, and enhanced horizontal orientation factors. The success of these emitters highlights the effectiveness of the molecular design strategy for advanced MR-TADF emitters and confirms their potential in high-performance optoelectronic devices. The study also revealed that the B–N bond-based fusion units enhance molecular rigidity and reduce structural vibrations, leading to improved RISC rates and spin-orbit coupling effects. The devices based on these emitters exhibited low turn-on voltages, high luminance, and excellent electroluminescence performance, with BCzBN-3B-based devices achieving the highest efficiency for TADF emitters in the binary emitting system. The results demonstrate the potential of this strategy for further advancements in MR-TADF emitters and narrowband OLEDs.A B–N covalent bond-involved π-extension strategy was developed to construct multi-boron-embedded multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters, enabling high-performance narrowband electroluminescence. The strategy involves post-functionalization of MR frameworks to generate high-order B/N-based motifs, leading to extended π-systems that enhance molecular rigidity and promote reverse intersystem crossing (RISC). This results in ultra-narrowband emitters with improved photophysical properties, such as high photoluminescence quantum yields and reduced emission linewidths. The developed emitters, DABNA-3B and BCzBN-3B, achieved external quantum efficiencies (EQE) of up to 42.6% in narrowband OLEDs, with significantly reduced efficiency roll-off at high brightness. These emitters demonstrated exceptional performance, including narrow emission bands (as narrow as 8 nm in n-hexane), high Φ_PL values, and enhanced horizontal orientation factors. The success of these emitters highlights the effectiveness of the molecular design strategy for advanced MR-TADF emitters and confirms their potential in high-performance optoelectronic devices. The study also revealed that the B–N bond-based fusion units enhance molecular rigidity and reduce structural vibrations, leading to improved RISC rates and spin-orbit coupling effects. The devices based on these emitters exhibited low turn-on voltages, high luminance, and excellent electroluminescence performance, with BCzBN-3B-based devices achieving the highest efficiency for TADF emitters in the binary emitting system. The results demonstrate the potential of this strategy for further advancements in MR-TADF emitters and narrowband OLEDs.