The adverse effects of parasitic gastrointestinal diseases on poultry production are enormous, especially where birds are kept under extensive or semi-intensive systems, but the diseases are rarely recognized as primary problems except during necropsy examinations. A two-year longitudinal cohort prospective study was conducted from October 2018 to September 2020 to determine the prevalence and diagnostic features of parasitic gastrointestinal diseases of poultry in the Veterinary Teaching Hospital, University of Jos, Plateau State, Nigeria. A total of 2,989 cases were diagnosed over the study period, out of which 749 (25.1%) were parasitic gastrointestinal diseases. Protozoan gastrointestinal diseases were more prevalent among cases of parasitic gastrointestinal diseases with a total of 610 (81.4%), while gastrointestinal helminths had 139 (18.6%) cases. The prevalence of helminthiasis was more in the months of August, September, December, January and February, while gastrointestinal protozoan diseases were more in the months of April, May, June, July, October and November. Helminths species commonly diagnosed were Ascaridia galli (8.3%); Heterakis gallinarum (4.7%); Capillaria spp (1.9%); Tetramere spp. (0.5%), Gongylonema spp. (0.1%), Strongyloides avium (0.1%), Raillietina spp (1.5%), Hymenolepis carioca (1.1%) and Davainea spp. (0.4%). The intestinal protozoan parasites of poultry predominantly diagnosed were Eimeria spp. (78.0%) but Histomonas meleagridis (2.7%), Trichomonas spp (0.7%) and Hexamita spp. (0.1%) were also seen. It is concluded that the prevalence of parasitic gastrointestinal diseases of poultry was high and could be some of the leading causes of low productivity and mortality in the study area even in commercial poultry farms under intensive production systems. It is recommended that attention should be given to good litter management where birds are reared on the floor to prevent the multiplication of the developmental stages of the gastrointestinal parasites in the environment, as well as the adoption of other control strategies against parasitic gastrointestinal diseases of poultry.