San Isidro is a partido of Buenos Aires Province, Argentina found in the north of Greater Buenos Aires. Its capital is San Isidro . It is 20km from the City of Buenos Aires.

The founder of San Isidro city was Domingo de Acassuso, who built a cathedral dedicated to San Isidro Labrador, having seen him in a dream. Other towns in the municipality are Martínez, Acassuso, Boulogne and Beccar.

The area of the partido is 51,44 km². In 2001 there were 293,000 inhabitants, representing 0.8% of the population of Argentina. San Isidro borders on the partidos of Tigre, Vicente López, San Martín y San Fernando. Until March 2004, the Mayor was Gustavo Posse.

Amongst its many sporting institutions, it is worth noting two of the most important rugby clubs in the country - SIC (San Isidro Club) and CASI (Club Atlético San Isidro). There are also sailing clubs on the Rio de la Plata.