Mar Menor (Spanish pronunciation: [ˈmar meˈnor], "Minor sea") is a salty lagoon, in the south-east of the autonomous community of Murcia, in Spain, separated from the Mediterranean sea by La Manga, a sandbar 22 km in length and with a width ranging from 100 to 1200 m.

It belongs to four municipalities: Cartagena, Los Alcázares, San Javier and San Pedro del Pinatar. With a surface area of nearly 170 km², a coastal length of 70 km, and warm and clear water no more than 7 m in depth, it is "the largest swimming pool in the world", in the opinion of famous swimmer and Hollywood actress Esther Williams.

Its relatively high salinity, which aids flotation, and remarkable sporting infrastructures make this one of the most popular places in Europe for a wide variety of water sports.

First the Phoenicians and then the Moorish kings, chose this "small sea" as the site of their summer residences. Today, this area attracts all those who seek a quiet place to rest and relax, with ideal weather conditions all year round.