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Despite Sn‐based perovskite solar cells (PSCs) prevailing over lead‐free candidates, the Sn vacancies (VSn) and Sn4+ defects seriously deteriorate device photovoltaic performance. The presently reported methods can only effectively achieve surface defect passivation, and it is of great challenge and fundamental importance to develop efficient strategy to deal with the intrinsic defects located inside the lattice. Herein, a novel bulk defect suppression strategy is proposed, introducing large organic piperazine cations (PZ2+) into the lattice of 3D FASnI3 perovskite to restrain the generation of bulk defects. The incorporation of PZ2+ results in forming a FA1−2yPZ2ySn1−yI3 (0 ≤ y ≤ 0.25) structure with no reduction in dimensionality, which guarantees the continuity of [SnI6] octahedral structures with unobstructed carrier transport and reduced charged defects. The potent interactions between PZ2+ and [SnI6] structures enhance VSn formation energy and effectively suppress bulk defect formation. As a result, the FASnI3+1%PZ films exhibit optimized crystalline quality, decreased background carrier density, lower p‐type self‐doping, and reduced trap state density. Benefiting from the above advantages, the FASnI3+1%PZ device achieves an optimal PCE of 9.15% and unencapsulated device maintains over 95% of initial PCE after aging for 1000 h in N2 golvebox. The bulk defect suppression strategy provides fire‐new building bricks toward high‐performance Sn‐based PSCs. [ABSTRACT FROM AUTHOR]