The 21st parallel south is a circle of latitude that is 21 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America.
Starting at the Prime Meridian and heading eastwards, the parallel 21° south passes through:
| Coordinates | Country, territory or ocean | Notes | |
|---|---|---|---|
| Atlantic Ocean | |||
| Indian Ocean | Mozambique Channel | ||
| Indian Ocean | |||
| Island of Réunion | |||
| Indian Ocean | Passing just south of Barrow Island, Western Australia, | ||
| Western Australia Northern Territory Queensland | |||
| Pacific Ocean | Coral Sea Passing through 's Coral Sea Islands Territory | ||
| Pacific Ocean | Coral Sea | ||
| Lifou Island | |||
| Pacific Ocean | Passing just north of Tongatapu island, Passing just north of Rarotonga island, Passing just south of Anuanuraro atoll, Passing just north of Anuanurunga atoll, Passing just north of Nukutepipi atoll, | ||
| Mato Grosso do Sul São Paulo State Minas Gerais Rio de Janeiro State Espírito Santo | |||
| Atlantic Ocean |