The study aims at determination of the rainy season period and an analysis of rainfall variability in the upstream lower Mouhoun, a sub-catchment of the Mouhoun River in Burkina Faso during the period 1980-2020. Daily rainfall data from fourteen stations covering the period 1980-2020 were collected and analyzed as part of this study. The duration of the rainy season was determined using a statistical approach. Mann Kendall and Pettitt tests are used to assess the significance of trends and breaks in annual rainfall. Determination of the standardized rainfall index was used to determine the evolution of seasonality. An analysis of the return of several rainfall indices was carried out to determine the most significant ones for the majority of stations. Analysis of annual rainfall trends over the period revealed alternating wet and dry phases, with a general upward trend in cumulative annual rainfall between 1980 and 2020 during the wet season. Annual rainfall rose by 10 to 12.35 % after the breaks year at most stations. At the stations of Boromo, Didyr, Imasgo, Tiogo, and Toma, the identified break years are 1992, 2001, 2002, 1990 and 1993, respectively. The return of the wet season is more pronounced between 1990 and 2000. The length of the rainy season is decreasing at most stations, despite an increase in the number of rainy days. There has been an increase in the frequency of high-intensity rain (over 50 mm/day) and low-intensity rain (under 5 mm/day). This upsurge in high-intensity rainfall is conducive to flooding and has a negative impact on farmers' speculative crops in the basin.