| Tallinn School No. 21 | |
| Native Name: | Tallinna 21. Kool |
| Streetaddress: | Raua 6 |
| Coordinates: | 59.4354°N 24.7664°W |
| Type: | Public |
| Established: | 1903 |
| Principal: | Meelis Kond |
| Grades: | 1–12 |
| Pupils: | 1385 |
Tallinn School No. 21 is a secondary school in Tallinn, Estonia. The school was founded as the Tallinn kroonualgkool No. 21. The first schoolhouse was a wooden house on J. Poska 6a. In the 1923/24 academic year, as the number of students grew, a new schoolhouse was needed.
The school is a member of the G5 Union of Schools, which comprises what some call the "elite Tallinn city centre schools": Tallinn English College, Tallinn French School, Gustav Adolf Grammar School, and Tallinn School No. 21.
The schoolhouse is at Raua 6. The architect was . From 2001 to 2003, the schoolhouse was renovated.
The school provides both primary and secondary education. In the secondary stage (gymnasium) students can choose between three tracks: science, English, and social and humanities studies.