| Country: | DEU |
| Type: | A |
| Route: | 21 |
| Map Custom: | yes |
| Length Km: | 50 |
| States: | Schleswig-Holstein, Lower Saxony (planned) |
| Previous Type: | A |
| Previous Route: | 20 |
| Next Type: | A |
| Next Route: | 23 |
It runs from an interchange with the A 1 at Bargteheide to Wankendorf, halfway between Bad Segeberg and Kiel. A northbound extension to Kiel and a southbound extension connecting to the A 39 are being planned. The A 21 will cross the extension of the A 20 near Bad Segeberg.