Mar de Ajó is a coastal city in Buenos Aires Province, Argentina, and is located in the southern end of the seaside La Costa Partido (the Coast District).

[1] It owes its name to a word used by the Guarani aborigines who inhabited the area, whose meaning is Soft Mud.

During the arrival in 1580 of the Paraguayan Hernando Arias de Saavedra to the shores of the mouth of the Río de la Plata with the Atlantic Ocean, his expedition made use of the collaboration of a group of evangelized Guarani indigenous people, who identified this area with the word Tuju, a term in Guarani that indicated the presence of white mud on the coasts and bottom of the river, for this reason, this place ended up being known as the Rincón del Tuyú.

There's the Ria Ajó, which was identified by the Guarani with the term Ajo, which alluded to the soft white mud of its bed, which made the terrain unstable when it came to traversing it on foot.

Other notable installations include the Rubén Luis Di Palma Regional Racetrack (among the venues of the TC2000 rally), the Historical Museum and Archives (featuring glyptodons and other cenozoic era fossils), the Parish of Santa Margarita María de Alacoque, and the ship graveyard at Nueva Atlantis – site of the shipwreck of numerous 19th-century merchant vessels, notably the Margaretha, an Imperial German ship that ran aground in 1880, and whose remains appear during low tide.