In this study, a donor–π-acceptor-type thermally activated delayed fluorescence (TADF) emitter (BzITz) was designed and synthesized using 5H-benzo[d]benzo[4,5]imidazo[1,2-a]imidazole as a fused rigid electron donor, and a benzonitrile merged triazine unit as an electron acceptor. The effects of the fused rigid donor unit on the photophysical and electroluminescence properties of the TADF emitter were thoroughly explored. Compared with a reported emitter based on carbazole in a previous study, photophysical analysis results showed that BzITz exhibited a relatively small singlet–triplet energy splitting, a short delayed fluorescence lifetime, and a high photoluminescence quantum yield. Consequently, an organic light-emitting diode fabricated using the BzITz emitter exhibited an improved device performance with an external quantum efficiency of 24.0%, a current efficiency of 49.4 cd A⁻¹, and a low-efficiency roll-off. These findings show that the use of fused rigid donors can play a critical role in the design of efficient TADF emitters.