LAUSR

2D Dion−Jacobson (DJ) perovskite single crystals (PSCs) usually demonstrate better X‐ray detection performance than Ruddlesden‐Popper (RP) PSCs. However, the mechanism of the improved performance is still elusive. Here, by the aid of strong interactions between dimethylbiguanide (DGA) and PbI2, a novel DJ‐perovskitoid (DGA)PbI4 is designed. From the comparison of (DGA)PbI4 to other 2D PSCs, it is discovered that the tiniest lattice distortion and increased hydrogen bonds in the atom‐scaled analysis strengthen lattice rigidity and weaken electron‐phonon coupling to suppress disordered scattering of carriers, resulting in significantly improved carrier transport and stability. Therefore, high carrier mobility (78.1 cm2 V−1 s−1) and a pronounced sensitivity of 4869.0 µC Gyair−1cm−2 are achieved using (DGA)PbI4, which are the best in 2D Pb‐based PSC devices to date. Finally, the (DGA)PbI4 devices exhibit good spatial resolution in X‐ray imaging and excellent long‐term stability to work as a promising candidate for medical diagnostics and nondestructive determination.