- Colonia is a city in the southwestern part of Uruguay. It is located on the northern shore of the Río de la Plata, across from Buenos Aires, Argentina. Colonia is known for its historic quarter, which is a UNESCO World Heritage Site. The city has a population of approximately 27,000 people and attracts many tourists due to its charming colonial architecture and cobblestone streets. Colonia offers a variety of attractions, including museums, art galleries, restaurants, and beautiful beaches along the river.