| C/1861 G1 (Thatcher) | |
| Discoverer: | A. E. Thatcher |
| Discovery Date: | April 5, 1861 |
| Designations: | 1861 I |
| Orbit: | Long period comet |
| Observation Arc: | 149 days |
| Obs: | 187 |
| Semimajor: | 56.3 AU |
| Perihelion: | 0.921 AU (1861) 0.917 AU (2283) |
| Aphelion: | 112 AU (beyond Eris) |
| Eccentricity: | 0.983 |
| Period: | 422 yr (barycentric) |
| Inclination: | 79.77° |
| Last P: | 1861-Jun-03 |
| Next P: | 2283 ±5 |
Comet C/1861 G1 (Thatcher) is a long-period comet with roughly a 422-year orbit that is expected to return around 2283. It was discovered by A. E. Thatcher. It is responsible for the April Lyrid meteor shower. Carl Wilhelm Baeker also independently found this comet. The comet passed about 0.335abbr=unitNaNabbr=unit from the Earth on 5 May 1861 and last came to perihelion (closest approach to the Sun) on 3 June 1861.
C/1861 G1 is listed as a long-period "non-periodic comet" because it has not yet been observed at two perihelion passages. When it is seen to come back around 2283, it should receive the P/ designation.
The comet is the parent body of the April Lyrids meteor shower.