La Picasa Lake, Freshwater lake spanning Buenos Aires, Santa Fe, and Córdoba provinces, Argentina.

La Picasa Lake is a shallow freshwater lake shared by Santa Fe and Buenos Aires provinces, with a basin reaching into Córdoba. Its average depth is about 3 metres, and a 16 km railway causeway of rough pale grey and brown rock crosses the water, carrying the San Martín line between Diego de Alvear and Aarón Castellanos stations.

The lake's water level rose from 104.5 m above sea level during the dry period of 1920–1973 to over 106 m in the wet period that began in 1973, expanding the flooded area from 40 km² to 300 km². This flooding submerged National Route 7 and the Ferrocarril General San Martín line, threatening more than 100,000 people with displacement. In 2008, a 16 km railway causeway (pedraplén) restored train traffic on the San Martín line across the water.

The lake gives its name to the San Martín railway causeway that crosses it, a 16 km stone embankment built in 2008 and repaired in 2021. Trains between Diego de Alvear and Aarón Castellanos stations run over the water, and in 2009 a passenger train crossed at 110 km/h.

The lake is accessible via National Route 7, with the nearby towns of Vedia and Rufino offering places to stay and eat. The shallow shores allow easy access to the water from many points along the edge.

The railway causeway crossing the lake, a 16 km stone embankment, reopened the San Martín line's freight traffic on 20 November 2008. On 30 December 2009 a passenger train crossed it at 110 km/h during the World March for Peace and Nonviolence. Repairs finished in October 2021.