Sn(II)-based halide perovskite semiconductor materials are promising for a variety of electronics and optoelectronics applications but suffer from poor intrinsic materials stability. Here, we report the synthesis and characterization of a stable Sn (II)-based two-dimensional perovskite featuring a pi-conjugated oligothiophene ligand, namely (4Tm)(2)SnI4, where 4Tm is 2-(3''',4'-dimethyl-[2,2' :5',2 '' :5 '',2'''-quaterthiophen]-5-yl) ethan-1-ammonium. The conjugated ligands facilitate formation of micrometer-size large grains, improve charge injections, and stabilize the inorganic perovskite layers. Thin film field-effect transistors based on (4Tm)(2)SnI4 exhibit enhanced hole mobility up to 2.32 cm(2) V(-1)s(-1) and dramatically improved stability over the previous benchmark material (PEA)(2)SnI4. Stabilization mechanisms were investigated via single-crystal structure analysis as well as density functional theory calculations. It was found that the large conjugated organic layers not only serve as thick and dense barriers for moisture and oxygen but also increase the crystal formation energy via strong intermolecular interactions. This work demonstrates the great potential of molecular engineering for organic-inorganic hybrid perovskite materials toward applications in high-performance electronics and optoelectronics.