Porto Velho is the capital of the Brazilian state of Rondônia, in the upper Amazon River basin. The population is 548,952 people. Located on the border of Rondônia and the state of Amazonas, the town is an important trading center for cassiterite, the mining of which represents the most important economic activity in the region, as well as a transportation and communication center. It is on the eastern shore of the Madeira River, one of the main tributaries of the Amazon River. It is also Rondônia's largest city, and the largest state capital of Brazil by area.

Rondônia is one of the 26 states of Brazil, located in the northern subdivision of the country between latitudes 07º 58' 32" and 13º 41' 36" S and longitudes 59º 46' 26" and 66º 48' 21"W. To the west is a short border with the state of Acre for 22 km, to the north is the state of Amazonas for about 930 km across the Serra de São Jão, latitude 7° 58ˈ south, and north of the Mapiri-Madeira River, in the east is Mato Grosso for about 820 km across the Serra Grande, Serra de Providência and the Rio Branco, Inue (Languiaru) and Tenente Marques-Eugênia, and in the south and southwest is Bolivia for 1,464 km across the Guaporé and Abuna Rivers. Rondônia has a population of 1,815,000 as of 2021. It is the fifth least populated state. Its capital and largest city is Porto Velho, bathed by the Madeira River. The state was named after Cândido Rondon, who explored the north of the country during the 1910s. The state, which is home to c. 0.7% of the Brazilian population, is responsible for c. 0.3% of the Brazilian GDP.