It was found in Buenos Aires' upscale Puerto Madero neighborhood, on the banks of the Plata River. The area used to be the city's old port, but was eventually filled in and developed.
Mayor Mauricio Macri announced the ship's find at a news conference Tuesday.
Urban archaeologist Marcelo Weissel says it probably dates back to 1750 or earlier. But he says it likely does not contain any treasure, beyond its archaeological value.
Experts will work carefully to rescue the ship so that it can be sent to a museum.