| C/1999 T1 (McNaught-Hartley) | |
| Discoverer: | Robert McNaught, Malcolm Hartley |
| Discovery Site: | Siding Spring Observatory |
| Discovery Date: | 7 October 1999 |
| Designations: | C/1999 T1, Comet McNaught-Hartley |
| Epoch: | 2451880.5 (2 December 2000) |
| Obs: | 704 |
| Eccentricity: | 0.99985 |
| Last P: | 13 December 2000 |
C/1999 T1 (McNaught-Hartley) is a near-parabolic long-period comet, discovered by Robert McNaught and Malcolm Hartley at the Siding Spring Observatory in 1999.
Research published in 2004 found that the Ulysses spacecraft had likely detected ions from the comet tail of C/1999 T1. This was the spacecraft's second encounter with a comet tail, after Comet Hyakutake in 1996.