A new family of indole-fused multi-resonance thermally activated delayed fluorescence materials have been developed, which exhibit narrowband blue emission with a small full width at half maximum of 22–23 nm. High maximum external quantum efficiencies of up to 16.8% have been achieved in organic light-emitting diodes. Furthermore, it is unveiled that peripheral substitutions can not only promote device efficiency but also help to maintain narrowband emission under high dopant concentrations.