Theodore Dwight Weld (November 23, 1803 – February 3, 1895) was one of the architects of the American abolitionist movement during its formative years from 1830 to 1844, playing a role as writer, editor, speaker, and organizer. He is best known for his co-authorship of the authoritative compendium American Slavery as It Is: Testimony of a Thousand Witnesses, published in 1839. Harriet Beecher Stowe partly based Uncle Tom’s Cabin on Weld's text; the latter is regarded as second only to the former in its influence on the antislavery movement. Weld remained dedicated to the abolitionist movement until slavery was ended by the Thirteenth Amendment to the United States Constitution in 1865.

According to Lyman Beecher, the father of Harriet Beecher Stowe, Weld was "as eloquent as an angel, and as powerful as thunder." His words were "logic on fire".

In 1950, Weld was described as being "totally unknown to most Americans".

His obscurity was of his own choosing. Weld would never accept an office of authority or honor in any antislavery organization. He refused to speak at antislavery conventions or anniversaries, or even to attend them if he could avoid it. He shunned the cities, and chose to labor in the country districts, where newspapers were few, and his activities were seldom reported except by abolition journals. His writings were published anonymously, and he would seldom allow the content of his speeches or his letters from the field to appear in print at all.

Weld was born in Hampton, Connecticut, the son and grandson of Congregational ministers. He was descended from Thomas Welde, one of the original trustees of Harvard College. His mother owned slaves. At age 14 Weld took over his father's hundred-acre (forty-hectare) farm near Hartford, Connecticut, to earn money to study at Phillips Academy in Andover, Massachusetts, attending from 1820 to 1822, when failing eyesight caused him to leave. After a doctor urged him to travel, he started an itinerant lecture series on mnemonics, traveling for three years throughout the United States, including the South, where he saw slavery first-hand. In 1825 Weld moved with his family to Fabius, in upstate New York. At the time of the Weld-Grimké marriage they were living in Manlius, New York.

Weld then (1825) attended classes at Hamilton College in Clinton, Oneida County, New York, though he did not enroll as a student and does not appear in the College's published lists of students. About 1825 he stayed at the College in the suite of tutor William Kirkland, and not only attended classes but was "something of a leader among the students". The famous evangelist Charles Grandison Finney was based in Oneida County, and according to him, Weld "held a very prominent place among the students of Hamilton College, and had a very great influence." He described himself as "educated at Hamilton College." However, Hamilton turned down his proposal of a manual labor program.

While a student Weld attended some of Finney's many revivals, for he became Finney's disciple. In Utica, intellectual capital of western New York, center of abolitionism, and county seat of Oneida County, he met and became a good friend of Charles Stuart, an early abolitionist, who at that time (1822–1829) was head of the Utica Academy. They spent several years as members of Finney's "holy band".

In the winter of 1827, he and his brother Charles worked on a whaling vessel in Labrador.