James Asheton Bayard Jr. (November 15, 1799 – June 13, 1880) was an American lawyer and politician from Delaware. He was a member of the Democratic Party and served as U.S. Senator from Delaware.

Bayard was born in Wilmington, Delaware, on November 15, 1799. He was a son of Nancy (née Bassett) Bayard and James A. Bayard, a member of the Federalist Party who served as U.S. Representative and U.S. Senator from Delaware. His older siblings included brother Richard H. Bayard, also a U.S. Senator from Delaware, and Ann Caroline Bayard, who operated the Bayard family's ironworks, Victoria Furnace, with their brother Henry Milligan Bayard.

His paternal grandparents were Dr. James Asheton Bayard and Ann (née Hodge) Bayard. The Bayard family was descended from a sister of Director-General Petrus Stuyvesant and came to Bohemia Manor, Cecil County, Maryland, in 1698. His maternal grandfather was Richard Bassett, a signatory to the United States Constitution and U.S. Senator from Delaware.

Bayard studied the law, and began his legal practice in the city of Wilmington. From 1836 until 1843 he served as United States Attorney for Delaware. In 1851 he was elected to the U.S. Senate. He was re-elected in 1857 and 1863, and served from March 4, 1851, to January 29, 1864, when he resigned. As U.S. Senator he was chairman of the Committee on Engrossed Bills in the 32nd Congress, a member of the Committee on Public Buildings in the 33rd Congress and 34th Congress, a member of the Committee on Judiciary in the 35th Congress and 36th Congress, and a member of the Committee on Public Buildings and Grounds in the 35th Congress.

Bayard served on the boards of various railroads, including the Wilmington and Susquehanna Railroad, the Philadelphia, Wilmington and Baltimore Railroad (for which service he is named on the 1839 Newkirk Viaduct Monument), and the Pennsylvania Railroad.

In 1846, Bayard represented slave owners in a civil suit against Thomas Garrett, a Wilmington iron merchant who was also a "stationmaster" on the Underground Railroad. The plaintiffs demanded damages from Garrett for helping around 10 slaves escape to freedom. The suit was tried in the U.S. District Court in New Castle, Delaware, before Chief Justice of the Supreme Court Roger B. Taney (sitting as a circuit judge). (Taney later issued the notorious Dred Scott decision as Chief Justice.) Bayard won a judgement that all but bankrupted Garrett, who declared on the spot that he would redouble his anti-slavery efforts: "Friend, I haven't a dollar in the world, but if thee knows a fugitive who needs a breakfast, send him to me".

Bayard was a conservative and adhered to his interpretation of tradition throughout the American Civil War. He believed the South should be allowed to secede peacefully, and privately hoped for the secession of Delaware and a state convention to address the issue. Citing property rights of owners, he opposed abolitionist measures. He also stated both his opposition to the Civil War and his opposition to any presidential or congressional acts used to suppress the independence of the Southern states.

During the Civil War, the Senate required all senators to swear an oath of loyalty to the Union. Bayard refused, stating that such an oath would be unconstitutional, and after taking the oath and giving a long speech disputing its legality, resigned from the Senate.