Uniform and high‐electronic‐quality perovskite thin films are essential for high‐performance perovskite devices. Here, it is shown that the 3‐(decyldimethylammonio)‐propane‐sulfonate inner salt (DPSI), which is a sulfonic zwitterion, plays dual roles in tuning the crystallization behavior and passivating the defects of perovskites. The synergistic effect of crystallization control and defect passivation remarkably suppresses pinhole formation, reduces the charge trap density, and lengthens the carrier recombination lifetime, and thereafter boosts the small‐area (0.08 cm2) planar perovskite device efficiency to 21.1% and enables a high efficiency of 18.3% for blade‐coating large‐area (1 cm2) devices. The device also shows good light stability, which remains at 88% of the initial efficiency under continuous unfiltered AM 1.5G light illumination for 480 h. These findings provide an avenue for simultaneous crystallization control and defect passivation to further improve the performance of perovskite devices. The sulfonic zwitterion combines the functions of morphology tailoring and defect passivation together into one kind of functional molecule, and this “all‐in‐one” system provides a facile but effective pathway for the fabrication of high‐performance perovskite solar cells.