Porosity determines the storage capacity of shale reservoirs and is of great significance for evaluating shale gas resources and production. However, compared with commercially developed marine shales, paralic shales contain different organic matter types, mineralogical compositions, and microstructures. Moreover, a systematic understanding of the pore evolution mechanisms in paralic shales is lacking. Thus, we selected the overmature upper Permian Longtan Formation in northern Guizhou Province, southwestern China, as an example to investigate the pore characteristics and evolution mechanisms in paralic shales. Reflected-light microscopy with oil immersion combined with scanning electron microscopy observations confirmed that the macerals of the Longtan shales are composed mainly of vitrinite and pyrobitumen, followed by inertinite. The pore types can be divided into organic matter pores and mineral matrix pores. Organic matter pores include primary organic matter pores and secondary organic matter pores. Mineral matrix pores include intergranular and intragranular pores. Intragranular pores can be further divided into intraplatelet pores within clay aggregates, intercrystalline pores, and dissolution pores. Mesopores and macropores provide most of the total pore volume, whereas micropores provide most of the total surface area. Total organic carbon content is the main factor controlling the pore development, and the contribution of clay minerals to porosity is still questionable. The maceral types and thermal evolution are of great significance to the development of organic matter pores in paralic shales. The primary composition and diagenetic modifications of the identified four major shale lithofacies are different, and therefore, result in various pore networks of each lithofacies.