The construction of highly active, durable, and cost‐effective catalysts is urgently needed for green hydrogen production. Herein, catalysts consisting of high‐density Pt (24 atoms nm−2) and Ir (32 atoms nm−2) single atoms anchored on Co(OH)2 were constructed by a facile one‐step approach. Remarkably, Pt1/Co(OH)2 and Ir1/Co(OH)2 only required 4 and 178 mV at 10 mA cm−2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. Moreover, the assembled Pt1/Co(OH)2//Ir1/Co(OH)2 system showed mass activity of 4.9 A mgnoble metal−1 at 2.0 V in an alkaline water electrolyzer, which is 316.1 times higher than that of Pt/C//IrO2. Mechanistic studies revealed that reconstructed Ir−O6 single atoms and remodeled Pt triple‐atom sites enhanced the occupancy of Ir−O bonding orbitals and improved the occupation of Pt−H antibonding orbital, respectively, contributing to the formation of the O−O bond and the desorption of hydrogen. This one‐step approach was also generalized to fabricate other 20 single‐atom catalysts.