| Country: | HUN |
| Type: | MR |
| Route: | 21 |
| Map: | 21-es főút-térképe.png |
| Length Km: | 65 |
| Length Round: | 1 |
| Terminus A: | near Hatvan |
| Junction: |
|
| Terminus B: | Somoskőújfalu border with |
| Counties: | Heves, Nógrád |
| Cities: | Hatvan, Lőrinci, Pásztó, Bátonyterenye, Salgótarján |
The Main road 21 is a south–north direction First class main road in the road in the valley of a Zagyva river, that connects the M3 motorway's Hatvan junction to the, facilitating access from the capital city of Hatvan to Salgótarján-Somoskőújfalu. The road is 65 km long.[1]
The road, as well as all other main roads in Hungary, is managed and maintained by Magyar Közút, state owned company.