Tourism exerts a significant pressure on water resources in tropical islands and is the main reason for the increase in water demand. This paper analyzes and quantifies water demand in Santa Cruz, the most populated island of Galápagos Archipelago (Ecuador). The data for this study were collected from a survey of 374 households in the two main settlements, Puerto Ayora and Bellavista. Water supply from different sources was assessed and compared with domestic and tourist water use. The study revealed that the daily average water demand per capita differs 40% between the settlements, mainly resulting from a difference in water tariffs. The tourist sector was also confirmed as the major consumer, accounting for 55% of the total water demand. Lastly, implementation of proper water demand management measures is essential in order to develop consumers’ awareness and sustainable tourism.