A class of unsymmetric rod-like organic molecules, which simply combine extended π-electron skeletons (π-cores) with straight-chain alkyl groups, exhibits a remarkable tendency to self-organize into crystalline molecular layers—referred to as layered crystallinity. These molecules facilitate the formation of highly uniform, two-dimensional organic semiconductor (OSC) thin films through simple solution processing, which enables the production of high-performance printed organic field-effect transistors (OFETs). In this chapter, we first illustrate the unique appearance of these crystals to characterize the layered crystallinity. We then explore the origins of this crystallinity using crystal structure analysis and intermolecular interaction calculations, with particular emphasis on how these characteristics are influenced by the length of the alkyl chains. Next, we discuss the development of various layered-crystalline OSCs, achieved through strategic molecular design modifications of both the π-cores and substituents, while maintaining their high degree of layered crystallinity. Finally, we highlight the enhanced device performance with showing high carrier mobility and sharp on/off switching at low voltages, achieved by forming exceptionally clean semiconductor–insulator interfaces that fully leverage the layered crystallinity.