Mar Menor (, "minor/smaller sea") is a coastal saltwater lagoon in the Iberian Peninsula located south-east of the Autonomous Community of Murcia, Spain, near Cartagena.
Its name is the opposite of the Mediterranean, which is the  (greater/larger sea) of the region.
Four municipalities border the Mar Menor: Cartagena, Los Alcázares, San Javier and San Pedro del Pinatar.
With a surface area of 135 km, 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"), 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: Perdiguera islet, Mayor or El Barón islet, Ciervo islet, Redonda islet and del Sujeto islet.
Its relatively high salinity (which aids flotation), low waves, and remarkable sporting infrastructures makes it a popular place for a wide variety of water sports.