AbstractIn inverted perovskite solar cells, conventional planar 2D/3D perovskite heterojunctions typically exhibit a type‐II band alignment, where the electric field tends to drive the electron motion in the opposite direction to the direction of electron transfer. Here, a 2D/3D gradient heterojunction is developed by allowing the 2D perovskite to infiltrate the 3D perovskite surface along the grain boundaries using the interaction between the organic cation of the 2D perovskite and the pseudohalogen thiocyanate ion (SCN−), which has the ability to diffuse downward. The infiltrated 2D perovskite not only fills the gaps of grain boundaries with improved structural stability, but it also reconstructs the original landscape of the electric field toward the n‐doped surface to enable more rapid electron transfer and weaken the adverse type‐II band alignment effect. Since 2D perovskite seals the GBs, the nonvolatile SCN− can accumulate at the top and bottom dual interfaces, releasing residual stress and significantly inhibiting nonradiative recombination. The device exhibits an excellent efficiency of 24.76% (certified 24.29%) and long‐term stability that is >90% of the original PCE value after 800 h of heating at 85 °C or in high humidity (≈65%).