Ossian Haven Sweet (/ˈɒʃən/ OSH-ən; October 30, 1895 – March 20, 1960) was an African-American physician in Detroit, Michigan. He is known for being acquitted of murder in 1925 after he and his friends used armed self-defense against a hostile white mob protesting after Sweet moved into their neighborhood.

Born in Florida to a farming family, Sweet went to Wilberforce University for preparatory work and his undergraduate degree. He earned his medical degree from Howard University, also a historically black university. After moving into a white neighborhood, the Sweets had stones thrown at their house, breaking windows. Shots were fired, and one white man was killed and another wounded. Sweet, his wife, and nine associates at the house (including two brothers) were all arrested and charged with murder.

At the first trial, the jury could not agree on verdicts for several defendants. The judge declared a mistrial. The court accepted the defense motion to sever the defendants, and the prosecutor decided to first try Henry Sweet, Ossian's youngest brother. After the all-white jury acquitted Henry Sweet, the prosecutor declined to prosecute the rest of the defendants and dismissed the charges against them. Collectively these were known as the Sweet Trials. The National Association for the Advancement of Colored People (NAACP) provided assistance for the defense of Sweet and his co-defendants, first hiring Charles H. Mahoney to represent the clients, then hiring the noted attorney Clarence Darrow which brought more national attention and media to the trial.

In the years after the trial in Detroit, his daughter Iva, wife Gladys, and brother Henry all died of tuberculosis, and he died by suicide in 1960 after a series of unsuccessful professional and business decisions that left him destitute.

Ossian Haven Sweet was born in 1895 in Orlando, Florida as the second son of Henry Sweet and Dora DeVaughn. In 1898, his father, Henry, bought a farm in Bartow, the county seat of Polk County, Florida, and moved there with his entire family. They lived in a small farmhouse, and the children worked with the farm animals and in the fields. The Sweets had a total of ten children, including his brother Henry; they lived in cramped quarters on what little money they could earn through their farm.

At age five, Ossian Sweet witnessed the lynching of a black male teenager, Fred Rochelle, who was burned to death by a white mob. According to Sweet's later account, he was out alone at night about a mile from home, where he watched from the bushes as Rochelle was burned. Sweet later could "recount it with frightening specificity: the smell of the kerosene, Rochelle's screams as he was engulfed in flames, the crowd's picking off pieces of charred flesh to take home as souvenirs".

In September 1909, at age 13, Sweet left Florida. His parents wanted their son to obtain an education in the North, beyond what had been provided in his segregated Florida schools. He was sent to Wilberforce University in Xenia, Ohio, the first college to be owned and operated by African Americans. It also had preparatory classes to ready students for college-level work.

Wilberforce University was established in 1855 by a collaboration of white and black Methodists. By the early 1860s, the Cincinnati Methodist Church had withdrawn support due to demands of the Civil War. The school struggled financially after most of its paying students, mixed-race children of white Southern planters, were withdrawn. The African Methodist Episcopal Church (AME) paid the debt and took over ownership and operation of the college.