The Mitidja watershed (1 300 km2), located in northern Algeria, has experienced a drought in the late 20th century. The purpose of this work is to characterize this episode and determine its spatial extension in the watershed through the study of rainfall data measured at eight stations. The analysis of the long rainfall series of the Hamiz dam reference station (1905-2010) using several statistical tests (Mann-Kendall, Pettitt, Buishand U, and the Bayesian procedure of Lee and Heghinian) revealed that this episode of drought occurred between 1973 and 2001 and caused a decrease in annual rainfall varying between 16% and 24% in the watershed. After this period of drought, a wet period settled in the Mitidja, causing annual rainfall to increase by 31.4%, particularly at the Réghaïa station.