High maximum external quantum efficiency (EQE ), small efficiency roll-offs, and long operational lifetime at practical luminances are three crucial parameters for commercialization of organic light-emitting diodes (OLEDs). To simultaneously achieve these goals, it is desirable to have the radiative decay rate constant (k ) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S →S radiative decay rate constant (k ) and a small singlet-triplet energy gap (ΔE ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the ΔE while keeping the k large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and k approaching 2 × 10 s in thin films. The vacuum-deposited green OLEDs based on 1 and 4 demonstrate maximum EQEs of up to 24 and 27% with efficiency roll-offs of 5.5 and 2.2% at 1000 cd m , respectively; the EQEs maintain high at 10 000 cd m (19% (1) and 24% (4)). A long LT device lifetime of 1820 h at 1000 cd m for complex 1 is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.