Benjamin Franklin Butler (November 5, 1818 – January 11, 1893) was an American major general of the Union Army, politician, lawyer, and businessman from Massachusetts. Born in New Hampshire and raised in Lowell, Massachusetts, Butler was a political major general of the Union Army during the American Civil War and had a leadership role in the impeachment of U.S. president Andrew Johnson. He was a colorful and often controversial figure on the national stage and on the Massachusetts political scene, serving five terms in the U.S. House of Representatives and running several campaigns for governor before his election to that office in 1882.

Butler, a successful trial lawyer, served in the Massachusetts legislature as an antiwar Democrat and as an officer in the state militia. Early in the Civil War he joined the Union Army, where he first gained renown when he refused to return escaped slaves, designating them as contraband of war, an idea that the Lincoln administration endorsed and that played a role in making emancipation an official war goal. Later in the war, he was noted for his questionable military skills and his controversial command of New Orleans, which made him widely disliked in the South and earned him the "Beast" epithet. His commands were marred by financial and logistical dealings across enemy lines, some of which may have taken place with his knowledge and to his financial benefit.

At the request of General-in-Chief Ulysses S. Grant, President Abraham Lincoln relieved Butler from the posts he held in the Union Army after his failure in the First Battle of Fort Fisher, but he soon won election to the United States House of Representatives from Massachusetts. As a Radical Republican he considered President Johnson's Reconstruction agenda to be too weak, and he advocated harsher punishments of former Confederate leadership and stronger stances on civil rights reform. He was also an early proponent of impeaching Johnson. After Johnson was impeached in early 1868, Butler served as the lead prosecutor among the House-appointed impeachment managers in the Senate impeachment trial proceedings. Additionally, as Chairman of the House Committee on Reconstruction, Butler authored the Ku Klux Klan Act of 1871 and coauthored the landmark Civil Rights Act of 1875.

In Massachusetts, Butler was often at odds with more conservative members of the political establishment over matters of both style and substance. Feuds with Republican politicians led to his being denied several nominations for the governorship between 1858 and 1880. Returning to the Democratic fold, he won the governorship in the 1882 election with Democratic and Greenback Party support. He ran for president on the Greenback Party and the Anti-Monopoly Party tickets in 1884, having unsuccessfully sought the Democratic nomination as well.

Benjamin Franklin Butler was born in Deerfield, New Hampshire, the sixth and youngest child of John Butler and Charlotte Ellison Butler. His father served under General Andrew Jackson at the Battle of New Orleans during the War of 1812 and later became a privateer, dying of yellow fever in the West Indies not long after Benjamin was born. He was named after Founding Father Benjamin Franklin. His elder brother, Andrew Jackson Butler (1815–1864), served as a colonel in the Union Army during the Civil War and joined him in New Orleans. Butler's mother was a devout Baptist who encouraged him to read the Bible and prepare for the ministry. In 1827, at the age of nine, Butler was awarded a scholarship to Phillips Exeter Academy, where he spent one term. He was described by a schoolmate as "a reckless, impetuous, headstrong boy", and regularly got into fights.

Butler's mother moved the family in 1828 to Lowell, Massachusetts, where she operated a boarding house for workers at the textile mills. He attended the public schools there, from which he was almost expelled for fighting, the principal describing him as a boy who "might be led, but could not be driven." He attended Waterville (now Colby) College in pursuit of his mother's wish that he prepare for the ministry, but eventually rebelled against the idea. In 1836, Butler sought permission to go instead to West Point for a military education, but he did not receive one of the few places available. He continued his studies at Waterville, where he sharpened his rhetorical skills in theological discussions and began to adopt Democratic Party political views. He graduated in August 1838. Butler returned to Lowell, where he clerked and read law as an apprentice with a local lawyer. He was admitted to the Massachusetts bar in 1840 and opened a practice in Lowell.

After an extended courtship, Butler married Sarah Hildreth, a stage actress and daughter of Dr. Israel Hildreth of Lowell, on May 16, 1844. They had four children: Paul (1845–1850), Blanche (1847–1939), Paul (1852–1918) and Ben-Israel (1855–1881). Butler's business partners included Sarah's brother Fisher and her brother-in-law, W. P. Webster.

In 1844, Butler was elected a member of the American Philosophical Society.