| J. T. Marryat Hornsby | |
| Constituency Mp1: | Wairarapa |
| Parliament1: | New Zealand |
| Term Start1: | 10 December 1914 |
| Term End1: | 19 December 1919 |
| Predecessor1: | Walter Buchanan |
| Successor1: | Alex McLeod |
| Term Start2: | 6 December 1905 |
| Term End2: | 17 November 1908 |
| Predecessor2: | Walter Buchanan |
| Successor2: | Walter Buchanan |
| Term Start3: | 6 December 1899 |
| Term End3: | 25 November 1902 |
| Predecessor3: | Walter Buchanan |
| Successor3: | Walter Buchanan |
| Birth Date: | 1857 3, df=yes |
| Birth Place: | Hobart, Tasmania |
| Death Place: | Carterton, New Zealand |
| Party: | Liberal |
John Thomas Marryat Hornsby (13 March 1857 – 23 February 1921), generally known as J. T. Marryat Hornsby, was a New Zealand politician of the Liberal Party from the Wairarapa. He was a newspaper editor and proprietor.
Hornsby was born at Hobart, Tasmania in 1857. His family moved to Nelson in 1874 and when the other family members returned to Hobart, he remained in New Zealand.
He was a Member of Parliament for the electorate in the 14th (1899–1902), 16th (1905–1908), and 19th (1914–1919) Parliaments.
He defeated Walter Clarke Buchanan (the Conservative sitting member from 1887) in 1899, but lost to him in 1902 and 1908. Buchanan was later the Reform Party candidate.
Hornsby was defeated in 1919 by Alex McLeod.
Hornsby died at Carterton in 1921.