| State: | ND |
| Type: | ND |
| Route: | 21 |
| Map Custom: | yes |
| Map Notes: | ND 21 highlighted in red |
| Length Mi: | 123 |
| Length Round: | 2 |
| Length Ref: | [1] |
| Established: | unknown |
| Direction B: | East |
| Junction: |
|
| Direction A: | West |
| Terminus A: | east-northeast of Amidon |
| Counties: | Morton, Grant, Hettinger, Slope |
| Previous Type: | ND |
| Previous Route: | 20 |
| Next Type: | ND |
| Next Route: | 22 |
North Dakota Highway 21 (ND 21) is an east-west highway in North Dakota. The eastern terminus is at ND 6 about 5miles north of Breien and the western terminus is at U.S. Route 85 (US 85) about 5miles east-northeast of Amidon. The highway is known for the large number of abandonments located along it.
The current ND-24 segment between ND-6 and the current ND-24/ND-1806 junction was originally part of ND-21. This segment was renumbered between 1950 and 1963.