Theodore Sedgwick III (January 27, 1811 – December 9, 1859) was an American attorney and writer on legal topics.

He spent the next several years traveling in Europe, including visits to Italy, Switzerland, France, and England.

In 1852, he became president of the Crystal Palace Association, organizing the construction of the building for the New York World's Fair.

[citation needed] In 1857, Sedgwick declined President James Buchanan's offers to become Minister to the Netherlands and assistant secretary of state.

His writings include his edition of the political writings of William Leggett (two volumes, 1840); Treatise on the Measure of Damages (1847; eighth edition, 1891); Treatise on the Rules which Govern the Interpretation and Application of Statutory and Constitutional Law (1857; second edition, 1874); and Thoughts on the Proposed Annexation of Texas (1844, originally published 1843 as letters to the New York Evening Post), which declared the annexation of Texas unconstitutional.