AbstractRecently, the mixed‐dimensional (3D/2D or 3D/1D) perovskite solar cells using small organic spacers have attracted interest due to their outstanding long‐term stability. Here, a new type of thiophene‐based organic cation 2‐(thiophene‐2yl‐)pyridine‐1‐ium iodide (ThPyI), which is used to fabricate mixed‐dimensional 3D/1D perovskite solar cells, is presented. The ThPyI‐based 1D perovskitoid is applied as a passivator on top of a 3D methyl ammonium lead iodide (MAPI) to fabricate surface‐passivated 3D/1D perovskite films or added alone into the 3D perovskite precursor to generate bulk‐passivated 3D MAPI. The 1D perovskitoid acts as a passivating agent at the grain boundaries of surface‐passivated 3D/1D, which improves the power conversion efficiency (PCE) of the solar cells. Grazing incidence wide‐angle X‐ray scattering (GIWAXS) studies confirm that ThPyI triggers the preferential orientation of the bulk MAPI slabs, which is essential to enhance charge transport. Champion bulk‐passivated 3D and surface‐passivated 3D/1D devices yield 14.10% and 19.60% PCE, respectively. The bulk‐passivated 3D offers favorable stability, with 84% PCE retained after 2000 h without encapsulation. This study brings a new perspective to the design of organic spacers having a different binding motif and a passivation strategy to mitigate the impact of defects in hybrid 3D/1D perovskite solar cells.