Municipality B, Administrative center in Cordón district, Montevideo, Uruguay.

Municipality B is one of the eight municipalities of Montevideo, Uruguay, covering the southern tip of the department on the Río de la Plata. It is the smallest and most densely populated municipality, made up of Ciudad Vieja, Centro, Cordón, Barrio Sur, Palermo, Parque Rodó and part of La Aguada. Avenida 18 de Julio is its axis, and it holds the seats of the three branches of state: the Torre Ejecutiva, the Palacio Legislativo and the Palacio Piria.

Municipality B was created on 17 December 2009 by departmental decree 33209, the first municipality of Montevideo, under the decentralisation law 18567; its creation was ratified by national law 18653 on 15 March 2010. It took in the communal centres 1 and 2 and electoral districts AAA to BKB. Silvana Pissano was mayor from 2020; Patricia Soria of the Frente Amplio has held the post since 2025.

Municipality B covers Ciudad Vieja, Centro, Cordón, Palermo, Barrio Sur, Parque Rodó and part of La Aguada, the ground of Montevideo's historic centre. Its axis is Avenida 18 de Julio, and the Rambla runs along its southern edge on the Río de la Plata. The neighbourhoods keep the city's oldest street layout and landmarks.

Municipality B covers Montevideo's southern tip along the Río de la Plata, from Bulevar Artigas and the Ramblas to the port. Avenida 18 de Julio crosses it, and the Ciudad Vieja, Centro, Cordón, Palermo, Barrio Sur, Parque Rodó and part of La Aguada lie within its limits. The municipal council sits in the Centro; the port and the three branches of government are nearby.

Municipality B contains the seats of all three branches of Uruguay's national government: the Torre Ejecutiva (Executive), the Palacio Legislativo (Legislative) and the Palacio Piria (Judicial). The Port of Montevideo, the country's main port, also lies within its boundaries, and Avenida 18 de Julio forms its central axis.