The 10-year-old will make his debut as the only actor seen on screen in Jon Favreau’s fresh take on the 1967 animated classic. The rest of the cast will be created using computer generation.
Sethi, from New York, was selected from thousands of children who auditioned worldwide in the US, Canada, the UK and New Zealand.
Sarah Finn, who chose Sethi, praised the young actor’s “ability to hold his own in any situation” and embody “the heart, humour and daring of the character”.
Stars signed up for The Jungle Book include Lupita Nyong’o as Rakcha; Ben Kingsley as panther Bagheera; Scarlett Johansson as python Kaa; and Idris Elba as villainous tiger Shere Khan.
The film, based on Rudyard Kipling’s stories about a child raised by wolves in the Indian jungle, is due to hit UK cinemas on 16 October 2015.
Warner Bros is working on a rival "darker" adaptation, steered by Dawn of the Planet of the Apes’ Andy Serkis.