Joseph Sidney Gelders (November 20, 1898 – March 1, 1950) was an American physicist who later became an antiracist, civil rights activist, labor organizer, and communist.

In September 1936, Gelders was kidnapped, beaten, and nearly killed by members of the Ku Klux Klan for his civil rights and labor organizing activities.

After his recovery, Gelders continued his activism and cofounded the Southern Conference for Human Welfare and the National Committee to Abolish the Poll Tax.

[8] The catalyst for his civil rights activism and labor organizing efforts was a 1934 ore-miner strike which led to the death of several black miners.

[7][10] At 11:30 pm on September 23, 1936, Gelders was on his way home from an International Labor Defense (ILD) meeting when he was kidnapped on a Birmingham street and flogged with a leather strap.

"[7][11] After a national outcry, Alabama Governor Bibb Graves, who had connections with the Ku Klux Klan, ordered a state police investigation and authorized a $200 reward (equivalent to US$4,391 in 2023).

[9] In the spring of 1938, Gelders went to Hyde Park, New York, to discuss his idea for the Southern Conference for Human Welfare (SCHW) with U.S. First Lady Eleanor Roosevelt and U.S. President Franklin D.

Gelders also obtained the support of William Mitch, the President of District 20 of the United Mine Workers, and the Alabama Director of the Congress of Industrial Organizations (CIO).

[13] A continuing organization was established as an outcome of this conference, and Gelders was designated executive secretary of the Civil Rights Committee.

He served in Coast Artillery Corps, First Company, in Mobile, Alabama, until transferring on October 1, 1918, to Fort Monroe for school.

[7] During World War II, Gelders reenlisted on March 30, 1942, as a technical sergeant in the Western Signal Corps, at Camp Kohler.

Their older daughter, Marge Frantz, was an activist, feminist, and among the first generation of academics who taught women's studies courses in the United States.

[20][21] The internal injuries that Gelders sustained from his kidnapping and beating in 1936 eventually led to his death on March 1, 1950, in San Francisco.