| Kosmos 1851 | |
| Mission Type: | Early warning |
| Cospar Id: | 1987-050A |
| Satcat: | 18103 |
| Mission Duration: | 4 years |
| Spacecraft Type: | US-K |
| Launch Date: | UTC |
| Launch Rocket: | Molniya-M/2BL |
| Launch Site: | Plesetsk Cosmodrome |
| Orbit Reference: | Geocentric |
| Orbit Regime: | Molniya |
| Orbit Inclination: | 62.8 degrees |
| Orbit Period: | 717.84 minutes |
| Apsis: | gee |
Kosmos 1851 (Russian: Космос 1851 meaning Cosmos 1851) is a Soviet US-K missile early warning satellite which was launched in 1987 as part of the Soviet military's Oko programme. The satellite is designed to identify missile launches using optical telescopes and infrared sensors.
Kosmos 1851 was launched from Site 43/4 at Plesetsk Cosmodrome in the Russian SSR. A Molniya-M carrier rocket with a 2BL upper stage was used to perform the launch, which took place at 07:40 UTC on 12 June 1987. The launch successfully placed the satellite into a molniya orbit. It subsequently received its Kosmos designation, and the international designator 1987-050A. The United States Space Command assigned it the Satellite Catalog Number 18103.