Mar Menor (Spanish pronunciation: [ˌmar meˈnor], "Minor sea" or "Smallest Sea". The Mediterranean Sea is also called "Largest Sea" [Mar Mayor] in the region) is a salty lagoon in the Iberian Peninsula located south-east of the Autonomous Community of Murcia, Spain, near Cartagena.
Four municipalities lie by the Mar Menor, 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 metres in depth, it is the largest lagoon in Spain.
The lagoon is separated from the Mediterranean Sea by La Manga ("the sleeve", in Spanish), a sandbar 22 km in length whose width ranges from 100 to 1,200 metres, with Cape Palos in its south-eastern vertex making for the lagoon's roughly triangular shape. There are five islets located within the lagoon, namely Perdiguera islet, Mayor islet, Ciervo islet, Redonda islet and del Sujeto islet.
Its relatively high salinity, which aids flotation, and remarkable sporting infrastructures makes it a popular place for a wide variety of water sports.
At the northern end there are salt-flats which include a wetland of international importance. This area is preserved as a natural park administered by the regional government. Its Spanish name is "". The microbes that live in this coastal lagoon have been recently described.