Indian Prime Minister Narendra Modi drove an inaugural cavalcade along the 4.9km Bogibeel bridge over the Brahmaputra river to inaugurate the project which has taken nearly two decades and $800m to complete.
The bridge, near the city of Dibrugarh, will enable the military to speed up the dispatch of forces to neighbouring Arunachal Pradesh, which borders China.
With the opening of the bridge, the rail journey from Dibrugarh to the Arunachal Pradesh capital Itanagar has been cut by 750km.
India lost territory in Arunachal Pradesh in a 1962 war with China.
The bridge has been designed to bear the weight of India’s heaviest 60-tonne battle tanks and so that fighter jets can land on it.
India opened its previous longest bridge, the 9.1km Dhola-Sadiya bridge, in 2017 to connect Assam and Arunachal Pradesh, also to boost tactical defences.
Construction of the Bogibeel bridge was agreed by the government as part of a 1985 agreement to end years of deadly agitation by Assamese nationalist groups.