It starts south east of Logroño at the Autovía LO-20 and runs parallel to the Autopista AP-68.

The road then continues south east between the Rio Ebro and the Imperial Channel of Aragon.

The road leaves the valley to the south, passing Iberian ruins and the Momumento al Tambor before reaching Alcañiz.

The N-420 continues north east, passing over the Coll del Moro to Tarragona.

The road follows the valley of the Rambla de Cervera and ends at the N-340 at Vinaròs 15 km north of Peníscola.