Hippopotamus is widely declining in several African countries, whereas its status and population size are still unknown in other countries, including South Sudan. We provide insights into population estimates, sex ratios, and age structures of the hippopotamus within Nimule National Park, South Sudan. Ground surveys and indirect counts were carried out for six days in 2020. Two line transects were walked, each of four km in length, using binoculars to identify their sex and age. We applied the Out-Bound Approach method to estimate the hippopotamus population size. A total of 190 individuals were estimated, but the confidence interval (Standard Error) of this estimate was too high to make this value satisfactorily accurate. An average population density of 0.463 individuals/km² was estimated. Notably, the adult sex ratio was significantly skewed towards females (1:1.5), calves comprised about 23% of the total number (n = 510). Four earlier unpublished studies (1983-2014) were compared with the data from this study. The results of this comparison showed a moderate increase in the hippopotamus population over the past 37 years, although with considerable fluctuations from survey to survey. Overall, Nimule National Park seems to house a stable hippopotamus population, that can be important for the conservation management planning for this species along the White Nile River course.