Global food demand is increasing rapidly as a result of multiple drivers including population growth, dietary shifts and economic development. Meeting the rising global food demand will require expanding agricultural production and promoting healthier and more sustainable diets. The goal of this paper is to assess and compare the water footprint (WF) of two recommended diets (Mediterranean and American), and evaluate the water savings of possible dietary shifts in two countries: Spain and the United States (US). Our results show that the American diet has a 29% higher WF in comparison with the Mediterranean, regardless of products’ origin. In the US, a shift to a Mediterranean diet would decrease the WF by 1629 L/person/day. Meanwhile, a shift towards an American diet in Spain will increase the WF by 1504 L/person/day. The largest share of the WF of both diets is always linked to green water (62%–75%). Grey water in the US is 67% higher in comparison with Spain. Only five products account for 36%–46% of the total WF of the two dietary options in both countries, being meat, oil and dairy products the food items with the largest WFs. Our study demonstrates that adopting diets based on a greater consumption of vegetables, fruits and fish, like the Mediterranean one, leads to major water savings.