Wide‐bandgap metal halide perovskites have demonstrated promise in multi‐junction photovoltaic (PV) cells. However, photoinduced phase segregation and the resultant low open‐circuit voltage (Voc) have greatly limited the PV performance of perovskite‐based multi‐junction devices. Here, we report a alloying strategy to achieve uniform distribution of triple cations and halides in wide‐bandgap perovskites by doping Rb+ and Cl− with small ionic radii, which effectively suppresses halide phase segregation while promoting the homogenization of surface potential. Based on this strategy, a Voc of 1.33 V was obtained from single‐junction perovskite solar cells, and a VOC approaching 3.0 V and a power conversion efficiency (PCE) of 25.0% (obtained from reverse scan direction, certified efficiency: 24.19%) on an 1.04‐cm2 photoactive area could be achieved in a perovskite/perovskite/c‐Si triple‐junction tandem cell, where the certification efficiency and stabilized power output (SPO) are by far the greatest PV performance of perovskite‐based triple‐junction tandem solar cells. This work overcomes the performance deadlock of perovskite‐based triple‐junction tandem cells by setting a materials‐by‐design paradigm.This article is protected by copyright. All rights reserved