Benjamin Constant Botelho de Magalhães (Niterói, 18 October 1836 – Rio de Janeiro, 22 January 1891) was a Brazilian military man and political thinker.
He was born at Niterói.
Primarily a positivist, influenced heavily by Auguste Comte, he was the founder of the positivist movement in Brazil (Sociedade Positivista do Brasil, Brazilian Positivist Society), and later this led to his republican views. He left the Brazilian Positivist society because of internal disagreements, but remained an ardent pupil of Comte until the end of his life.
Benjamin Constant had a difficult childhood and attempted suicide at the age of 12. He served in the War of the Triple Alliance, and had a large family. He felt underpaid and unhappy as a soldier. An intellectual at heart, he was a great supporter of Comte's Religion of Humanity.
His Republican views led him to found the Clube Militar (or Military Club), with Deodoro da Fonseca, in May 1887. It was based in the Praia Vermelha Military School, and, shortly before his death, overthrew the emperor and helped establish a republic.
A Brazilian town near the Amazon river was given his name.