San Nicolás is one of the neighbourhoods of Buenos Aires, Argentina, sharing most of the city and national government Neighborhood of Buenos Aires with neighboring Montserrat and home to much of the financial sector. It is referred usually as El Centro, and the part east of the 9 de Julio Avenue is known as Microcentro.

The Buenos Aires Central Business District is the main commercial centre of Buenos Aires, Argentina, though not an official city ward. While the barrios of Puerto Madero and Retiro house important business complexes and modern high-rise architecture, the area traditionally known as Microcentro is located within San Nicolás and Monserrat, roughly coinciding with the area around the historic center of the Plaza de Mayo. The Microcentro has a wide concentration of offices, service companies and banks, and a large circulation of pedestrians on working days. Another name given to this unofficial barrio is La City, which refers more precisely to an even smaller sector within the Microcentro, where almost all the banking headquarters of the country are concentrated.