Cubic Pd nanocrystals (CPNCs) as one of typical nanostructures are generally fabricated using I⁻ or Br⁻ as capping ions. However, which ion, I⁻ or Br⁻, exclusively mediates the growth of CPNCs in a given reaction system is not well understood. Herein, regardless of I⁻ or Br⁻ as the capping ion, we successfully achieved CPNCs in the same reaction system simply by adjusting the pH. Based on the Finke–Watzky kinetic model, an increase in pH accelerates the overall reduction rate of Pd²⁺, and the formation of CPNCs only occurs over the range of specific solution reduction rate constants (k₁). This kinetically illuminates that the reduction rate of Pd²⁺ is the physicochemical parameter that determines which ion, I⁻ or Br⁻, dictates the growth of CPNCs. Also, density functional theory (DFT) calculations further elucidate the dependence of the reduction rate of Pd²⁺ on pH and the configuration of the activated Pd²⁺ complex.