In West Africa, is located the Niger River Basin (NRB). Dry and wet conditions were investigated in this basin during the rainy (May-October) and dry (November-April) seasons, from 1980 to 2014. To do this was, calculated the Standardized Precipitation-Evapotranspiration Index (SPEI) at the time scale of 6-months for the whole NRB. The Lagrangian model FLEXPART v9.0 has been used to compute over the main semi-annual climatological moisture sources of the NRB, the budget of evaporation minus precipitation (E-P) over 10-day backward trajectories from the NRB itself. Positive (negative) (E-P) values indicate moisture uptake (loss). This permit evaluating the role of continental and oceanic sources of moisture separately for composites of extremely and severely dry and wet conditions in the basin. The results show for the dry season the negative trend of the April-SPEI6 values and the (E-P)>0 values obtained over the tropical east-north Atlantic Ocean (NAtl), the western Sahel and the Mediterranean region. Over these sources, the anomalies of (E-P) for driest and wettest composites indicate their direct response. On the contrary, for the rainy season, the October-SPEI6 values trend is positive, as well it occurs for the moisture uptake over the South Sahel (SSah) and the NRB itself. The anomalies of the (E-P) values for driest and wettest rainy seasons composites suggest a direct relationship with those obtained mainly over SSah, SAtl and the NRB itself.