Phosphorescent molecular aggregates show promise in realizing efficient and stable organic light‐emitting diodes (OLEDs) operating at high brightness level, which is highly desired for future lighting and display applications. Herein, four tetradentate Pd(II) complexes are prepared with judicious ligand design, and their electrochemical and photophysical properties are thoroughly examined. The studies indicate that slight structural changes of ligands can modify the hole and electron transporting capabilities, and alter the horizontal emitting dipole ratios of aggregates in amorphous film, the latter of which are sensitive to the thin‐film deposition conditions including the deposition rate and the choice of the templating layer. An optimized OLED device using Pd3O8‐Py5 aggregates exhibits a peak external quantum efficiency (EQE) of 37.3% and a reduced efficiency roll‐off with high EQEs of 36.0% and 32.5% at 1000 and 10 000 cd m−2, respectively. Moreover, such an efficient device demonstrates a long measured LT95 (time to 95% of the initial luminance) lifetime of over 500 h with an initial brightness of 17 304 cd m−2 corresponding to an estimated LT95 lifetime of 48 246 h at 1000 cd m−2. A tetradentate Pd(II) complex, i.e., Pd3O8‐Py5, exhibiting efficient aggregate emission is developed through judicious ligand design. The optimized organic light‐emitting diode device using Pd3O8‐Py5 aggregates exhibits a peak EQEair of 37.3% and a peak EQEair+sub of 67.4% with reduced efficiency roll‐off, and a long measured LT95 lifetime (time to 95% of the initial luminance) of over 500 h with an initial brightness of 17 304 cd m−2.