Istanbul is a megacity with a population of over 13 million. Due to experienced droughts and population growth in the city, interbasin water transfer from Melen Watershed was considered as the most feasible alternative to supply water that can compensate for water demand in the future. This decision changed the beneficial use of the Melen Watershed from irrigation water supply to drinking water supply and necessitated extensive pollution prevention measures to comply with the new water quality requirements. A Watershed Protection Action Plan was developed for the sustainable management of Melen Watershed. The action plan included measures concerning wastewater management, diffuse pollution management, solid waste management and water quality monitoring. In this paper, proposed protection actions in the plan are provided together with the activities realized so far. In addition, the changes made by the authorities during implementation of the plan are presented. The proposed Watershed Protection Action Plan as approved by the government is one of the first examples in Turkey, where watershed-based planning of water resources is implemented instead of administrative boundary-based management.