Theodore Sedgwick III (1811–1859) was an American attorney and author on legal topics.
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.