| John Fletcher Darby | |
| Order: | 4th Mayor of St. Louis, Missouri |
| Term Start: | April 14, 1835 |
| Term End: | October 31, 1837 |
| Predecessor: | John W. Johnson |
| Successor: | William Carr Lane |
| Term Start2: | April 14, 1840 |
| Term End2: | April 13, 1841 |
| Predecessor2: | William Carr Lane |
| Successor2: | John D. Daggett |
| State3: | Missouri |
| District3: | 1st |
| Term Start3: | March 4, 1851 |
| Term End3: | March 3, 1853 |
| Preceded3: | James B. Bowlin |
| Succeeded3: | Thomas H. Benton |
| Office4: | Member of the Missouri Senate |
| Term4: | 1838 |
| Birth Date: | December 10, 1803 |
| Birth Place: | Person County, North Carolina |
| Death Place: | Pendleton, Missouri |
| Party: | Whig |
| Spouse: | Mary Darby |
John Fletcher Darby (December 10, 1803May 11, 1882) was a U.S. Representative from Missouri and the fourth mayor of St. Louis.
Darby was born in Person County, North Carolina. He moved with his father to Missouri in 1818, where he worked on a farm before moving to Frankfort, Kentucky, in 1825. Thereafter, Darby studied law and was admitted to the bar. He then returned to Missouri to practice in St. Louis. He was the Mayor of St. Louis from 1835 to 1837 and 1840 to 1841, between which he served as a member of the Missouri Senate, in 1838. During his time as mayor, Darby was very active in getting the first Missouri railroad convention held in St. Louis. Eventually, that led to the incorporation of two railroads in the state. Lafayette Park was built, becoming the first city park west of the Mississippi River.
He was elected as a Whig to the Thirty-second United States Congress (March 4, 1851, to March 3, 1853), after which he returned to St. Louis and became a banker. He died near Pendleton, Missouri, on May 11, 1882, and his remains are interred at Calvary Cemetery in St. Louis.