Wide-bandgap (WBG) perovskite solar cells (PSCs) are important ingredients for tandem solar cells and play a crucial role in next-generation multijunction photovoltaics. Yet, the severe open-circuit voltage loss (V_loss) and stability have not been solved. Herein, a dimensionally graded 2D/3D heterostructure is fabricated by in situ fabricating a 2D FPEA₂PbI₄ capping layer on the surface of the 3D WBG perovskite film. Through this 2D/3D dimensionally graded design, an enhanced build-in potential promotes the oriented transport of photoinduced carriers and reduces the nonradiative recombination, leading to an ultrahigh open-circuit voltage of 1.31 V with a minimum V_loss of 0.43 V in a 1.74 eV WBG perovskite system and a desirable efficiency of 18.06%. A longer photoluminescence lifetime and decreased trap density indicate the reduced trap-assisted nonradiative recombination. Moreover, such a 2D/3D heterostructure exhibits enhanced stability under moisture and heat. This passivation strategy offers an effective approach to achieving high open-circuit voltage WBG PSCs by facile in situ dimensional engineering, which may pave a general way to step forward in achieving high-performance and stable WBG PSCs.