Isidor Rayner (b. April 11, 1850, d. November 25, 1912) was a Democratic member of the United States Senate, representing the State of Maryland from 1905-1912.

Rayner was born in Baltimore, Maryland and attended local private schools. He later attended the University of Maryland, Baltimore, and the University of Virginia. He began to study law and was admitted to the Maryland bar in 1871.

Rayner was elected to the Maryland House of Delegates and served from 1878-1884. In 1885, he was elected to the Maryland State Senate, serving one year until 1886.

Rayner was elected the same year to the Fiftieth United States Congress. He was an unsuccessful candidate for reelection in 1888 to the Fifty-first Congress, but was victorious in the next two elections to the Fifty-second and Fifty-third Congresses. He declined to be a candidate for renomination in 1894.

Several years later in 1899, Rayner was chosen to be the attorney general of Maryland, serving until 1903. He was elected as a Democrat in 1905 to the U.S. Senate, and was reelected again in 1911. While senator, he served as chairman of the Committee on Indian Depredations (Sixty-second Congress).

In 1912, Rayner died in Washington, D.C. while serving as senator. He is buried at Rock Creek Cemetery.