Lagoon Mirim is about 108 miles (174 km) long by 6 to 22 miles (9.7–35.4 km) wide and 62.25 km2 (24.03 sq mi). It is more irregular in outline than its larger neighbor to the north, Lagoa dos Patos, and discharges into the latter through São Gonçalo Channel, which is navigable by small boats. Lagoa Mirim has no direct connection to the Atlantic, but the Rio Grande, a tidal channel about 24 miles (39 km) long which connects Lagoon dos Patos to the Atlantic, affords an entrance to the navigable inland waters of both lagoons and several small ports. The southern part of the lake lies in Uruguayan territory, but its navigation, as determined by treaty, belongs exclusively to Brazil.
Both lagoons are the remains of an ancient depression in the coastline shut in by sand beaches built up by the combined action of wind and oceanic currents. They are at the same level as the ocean, but their waters are affected by the tides and are brackish only a short distance above the Rio Grande outlet.
The Taim Ecological Station lies to the east of the lagoon, in the narrow land strip between it and the Atlantic Ocean.
The Jaguarão/Yaguarón River, which forms part of the Brazil-Uruguay boundary line, empties into Lagoon Mirim, and is navigable 26 miles (42 km) up to and beyond the twin towns of Jaguarão (Brazil) and Rio Branco (Uruguay).
On the Uruguayan side apart from the Yaguarón River, the Cebollatí River and the Tacuarí River empty into the Lagoon Mirim. The amount of water received by the lagoon is greater from the Uruguayan side.