| C/1999 T1 (McNaught-Hartley) | |
| Discoverer: | Robert H. McNaught Malcolm Hartley |
| Discovery Site: | Siding Spring Observatory |
| Discovery Date: | 7 October 1999 |
| Designations: | Comet McNaught-Hartley |
| Epoch: | 2 December 2000 (JD 2451880.5) |
| Observation Arc: | 787 days (2.15 years) |
| Obs: | 661 |
| Aphelion: | 16,247 AU |
| Perihelion: | 1.172 AU |
| Semimajor: | 8,124 AU |
| Eccentricity: | 0.99985 |
| Period: | 732,246 years |
| Inclination: | 79.975° |
| Asc Node: | 182.483° |
| Arg Peri: | 344.758° |
| Tjup: | 0.234 |
| Earth Moid: | 0.19397 AU |
| Jupiter Moid: | 3.41621 AU |
| M1: | 8.3 |
| Rotation: | 1–10 days |
| Last P: | 13 December 2000 |
C/1999 T1 (McNaught–Hartley) is a near-parabolic long-period comet, discovered by Robert H. McNaught and Malcolm Hartley at the Siding Spring Observatory in 1999.
Comet McNaught–Hartley was a magnitude 15 object upon discovery on October 7, 1999. Gas emissions were measured in x-ray light by the Chandra observatory (alongside C/1999 S4 (LINEAR)) between 8–14 January 2001. Observations of its coma between January 26 and February 5, 2001 show that the nucleus has a rotation period between 1 and 10 days.
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.