Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA1−xDMAxPbI3 (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs. Secondary amine, dimethylamine is intentionally included in MAPbI3 perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. Solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% for NiOx‐based inverted perovskite solar cells and excellent operational stability under continuous light soaking.