It is widely recognized that the intrinsic charge transport properties in organic thin-film transistors (OTFTs) depend strongly on the crystal structure of the organic semiconductor layer. Pentacene, showing one of the highest charge carrier mobilities among organic semiconductors, is known to crystallize in at least four polymorphs, which can be distinguished by their layer periodicity d((001)). Only two polymorphs grow as single crystals, and their detailed crystal structure has been solved. The substrate-induced 15.4 A polymorph is the most relevant for OTFT applications; however, its crystal structure has remained incomplete as it only grows as a fiber structured thin film. Here we extend the crystal truncation rod X-ray scattering technique to fiber structured thin films. We determined the complete crystal structure of this polymorph grown on technologically relevant substrates. We found that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuning of intrinsic charge transport properties.