Wetlands are dynamic ecosystems with complex interrelationships of hydrology and ecology. They serve many critical functions and help control erosion and flooding. Among the various types of land cover in the basin, wetlands face the greatest threat at the moment. However, the basin-level geospatial distribution of wetlands remains poorly documented. This study aims to fill the gap in data and information on wetlands in the Koshi basin. It will serve as a tool that decision-makers can use to effectively allocate the limited resources for the conservation, management and restoration of wetlands in the basin. Wetlands database for the Koshi basin was developed using Landsat data of 1990 and 2010 through knowledge-based image classification techniques in ERDAS Imagine. Land and water mask (LWM) was considered an important index for wetland mapping. Statistics indicate that wetlands covered 5,391km2 of the Koshi basin in 1990 and 5,818km2 in 2010. The findings of this study will assist the efforts of other agencies and organizations to protect and restore wetlands and wetland-dependent wildlife.