The National Blue Trail (in Hungarian: Országos Kéktúra, short: OKT) is a national trail in Hungary, for most of its length part of the European Long Distance Walking Route E4. The route starts atop the Írott-kő Mountain (884 m) on the Austrian-Hungarian border then leads across Hungary eventually ending 1,170 km later at the village of Hollóháza by the Hungarian-Slovakian border. (Interactive map). The name of the Kéktúra (Blue Trail) is a reference to the marking of the path itself: it is a horizontal blue stripe on a white rectangle. All segments of the trail are freely accessible to the public; no fees have to be paid or permits obtained, there is only one ferry to take over the Danube between Visegrád and Nagymaros where you have to purchase a ticket. Although to have an official completion, you have to purchase a stamping brochure from MTSZ, the organizer of the trail.
During its course the Blue Trail visits arguably the most beautiful natural and man-made sights of Hungary, e.g. more than a dozen forts and castles, lookout towers, three World Heritage Sites of Hungary (the panorama of Budapest from the hills, the old village of Hollókő and the Stalactite Cave of Aggtelek), Lake Balaton, the Danube Bend, and the spent volcanoes of the Basin of Tapolca, etc.
According to the official website, as of March 2023 its total length is 1172.4 km and the total climb is 31 460 meters west to east. (Values vary as there are slight changes in the route changes every year.)