Estimating population densities is essential for understanding primate ecology and for guiding conservation efforts. Such information, however, is still scarce for many forest primates. The Djaffa Mountains guereza (Colobus guereza gallarum) is an Ethiopian endemic taxon for which information about their distribution and population size are missing. To fill this gap of knowledge, we conducted line-transect surveys in forest fragments within the range of C. g. gallarum in the Ahmar Mountains, Eastern Ethiopia. Between December 2020 and September 2021, we sampled 22 transects, covering a total distance of 93.7 km. We found considerable variation of encounter rates among the forest fragments. In total, we encountered 79 guereza clusters (most likely groups) and estimated their overall population density as 16.5 clusters/km2 (95%CI = 10.2–24.5) and 83.1 individuals/km2 (95%CI = 50.7–121.1). Their estimated population size was 15,205 (95%CI = 9288–22,163) individuals. This population density is thus lower than of C. g. guereza in other parts of Ethiopia. Given that the habitat of C. g. gallarum and its population are highly fragmented, further monitoring of the population and exploring the possibilities of reconnecting its habitat should be of conservation priority.