The Parnaíba Basin covers an area of 665,888 km2 between the northern and northeastern regions of Brazil. It is completely made up of siliciclastic sedimentary and volcanic rocks with ages varying between the Silurian and the Cretaceous. Among the formations of the Basin, the Tianguá Formation is considered a potential source rock. However, there are few geochemical and/or geological studies and no research work on the molecular parameters of the organic matter (OM) present in its outcropping rocks (OR). Thus, the objective of the present work is to characterize the OM, concerning the depositional paleoenvironment and the degree of thermal evolution, through molecular parameters and OM content. The OM was extracted using the Soxhlet system, chromatographed by classical methods, and analyzed using GC-MS and GC-MS/MS. The samples exhibited low total organic carbon values (TOC < 1) and the molecular parameters of saturated and aromatic hydrocarbons suggested a marine depositional environment, with algae, bacteria, and terrestrial plants input. The C30 biomarkers (24-n-propylcholestane), identified using synthetic standards and usually associated with the Chrysophyte algae, corroborated the marine origin of the depositional environment. In addition, some molecular parameters suggested little thermal evolution.