Isaac Leffler (November 7, 1788 – March 8, 1866), sometimes spelled Lefler or Loeffler, was an American lawyer who represented Virginia's 18th congressional district in the United States House of Representatives for one term in the 1820s; and served in the legislatures of the Commonwealth of Virginia and of the Wisconsin and Territories. He was the older brother of Iowa U.S. Representative Shepherd Leffler, who served in Congress twenty years later.
Born on his grandfather's plantation, "Sylvia's Plain," in Washington County, Pennsylvania, near Wheeling, Virginia (now West Virginia), Leffler attended the public schools and was graduated from Jefferson College, (now Washington & Jefferson College), in Canonsburg, Pennsylvania. After studying law, he was admitted to the bar and commenced practice in Wheeling. He served as member of the Virginia House of Delegates from 1817 to 1819, 1823 to 1827, 1832, and 1833. He served as member of the Virginia Board of Public Works in 1827.
In 1826, Leffler was elected as an Adams Party candidate to the Twentieth Congress, defeating incumbent Jacksonian Joseph Johnson. When running for re-election in 1828, he was defeated (along with President John Quincy Adams). Although Adams was defeated by Andrew Jackson, Leffler was defeated by Anti-Jacksonian Party candidate Philip Doddridge. In all, Leffler served in the U.S. House from March 4, 1827 to March 3, 1829.
In 1835, Leffler moved to that portion of Michigan Territory that is now Burlington, Des Moines County, Iowa, in Iowa's southeastern corner. He was admitted to the Des Moines County bar on April 15, 1835, and practiced law. While under Michigan's territorial governance, he was named as the chief justice of the first judicial tribunal of Des Moines County on April 11, 1836. After the creation of Wisconsin Territory on April 20, 1836, he served in the first legislature of the new Territory in 1836 and 1837, and served as Speaker of the House in 1837. After Iowa Territory was created from areas of Wisconsin Territory west of the Mississippi River in 1838, he served as member of the Iowa Territory house of representatives in 1841.
President John Tyler appointed Leffler as United States marshal for the district of Iowa on December 18, 1843. He served until removed by President James K. Polk on December 29, 1845, when he resumed the practice of law in Burlington. He declined the appointment of register of the land office at Stillwater (in what was then Minnesota Territory) in 1849. He was appointed by President Millard Fillmore as receiver of public moneys for the Chariton land district of Iowa on August 30, 1852, and served on that position until removed by President Franklin Pierce on March 29, 1853. He died in Chariton, Iowa, on March 8, 1866. He was interred in Aspen Grove Cemetery, in Burlington.