James Thomas Fields (December 31, 1817 – April 24, 1881) was an American publisher, editor, and poet. His business, Ticknor and Fields, was a notable publishing house in 19th century Boston.

He was born in Portsmouth, New Hampshire on December 31, 1817 and named James Field; the family later added the "s". His father was a sea captain and died before Fields was three. He and his brother were raised by their mother and her siblings, their aunt Mary and uncle George. At the age of 14, Fields took a job at the Old Corner Bookstore in Boston as an apprentice to publishers Carter and Hendee. His first published poetry was included in the Portsmouth Journal in 1837 but he drew more attention when, on September 13, 1838, he delivered his "Anniversary Poem" to the Boston Mercantile Library Association.

In 1839, he joined William Ticknor and became junior partner in the publishing and bookselling firm known after 1845 as Ticknor and Fields. Ticknor oversaw the business side of the firm while Fields was its literary expert. He became known for being likable, for his skill at finding creative talent, and for his ability to promote authors and win their loyalty. With this company, Fields became the publisher of leading contemporary American writers, with whom he was on terms of close personal friendship. He was also the American publisher of some of the best-known British writers of his time, some of whom he also knew intimately. The company paid royalties to these British authors, including Charles Dickens and William Makepeace Thackeray, at a time when other American publishers pirated the works of those authors. The first collected edition of Thomas de Quincey's works (20 vols., 1850–1855) was published by his firm. Ticknor and Fields built their company to have a substantial influence in the literary scene which writer and editor Nathaniel Parker Willis acknowledged in a letter to Fields: "Your press is the announcing-room of the country's Court of Poetry."

Sometime in 1844, Fields was engaged to Mary Willard, a local woman six years younger than him. Before they could be married, she died of tuberculosis on April 17, 1845. He maintained a close friendship with her family and, on March 13, 1850, married her 18-year-old sister Eliza Willard at Boston's Federal Street Church. Also sick with tuberculosis, she died on July 13, 1851. Grief-stricken, he left the United States for a time and traveled to Europe.

In 1854, Fields married Annie Adams, who was an author herself. Mrs. Fields was instrumental in helping Mr. Fields establish literary salons at their home at 37 Charles Street in Boston, where they entertained many well-known writers. One such writer was Nathaniel Hawthorne. After Hawthorne's death in 1864, Fields served as a pallbearer for his funeral alongside Amos Bronson Alcott, Ralph Waldo Emerson, Oliver Wendell Holmes Sr., Henry Wadsworth Longfellow, and Edwin Percy Whipple. In 1867, he performed the same role after the death of Nathaniel Parker Willis, along with Holmes, Longfellow, James Russell Lowell, and Samuel Gridley Howe.

Ticknor and Fields purchased The Atlantic Monthly for $10,000 and, about two years later in May 1861, Fields took over the editorship from Lowell. At a New Year's Eve party in 1865, he met William Dean Howells and 10 days later offered him a position as assistant editor of the Atlantic. Howells accepted but was somewhat dismayed by Fields's close supervision.

Ticknor died in 1864, leaving Fields the senior partner for the firm. Fields was less concerned with the retail store owned by the company and wanted to focus on publishing. On November 12, 1864, he sold the Old Corner Bookstore and moved Ticknor and Fields to 124 Tremont Street.

In 1868 the business became Fields, Osgood, and Company, recognizing James R. Osgood. On New Year's Day, 1871, Fields announced his retirement at a small gathering of friends.