A strategy for designing highly efficient thermally activated delayed fluorescence (TADF) emitters was reported. TADF emitters with donor–acceptor–donor (D–A–D)-type structures showed highly efficient TADF because of their small singlet–triplet energy gap and large oscillator strength. An organic light-emitting diode containing a D–A–D-type TADF emitter, cis-BOX2, exhibited a high external electroluminescence quantum efficiency of 17.6%.