The 10-year-old New York native beat out thousands of other boys for the role at open casting calls in New York City, New Zealand, London, and Canada. He has no previous acting experience.
"Casting is the most important element of any film and finding the right kid to play Mowgli was imperative," said director Jon Favreau in a statement. "Neel has tremendous talent and charisma. There is a lot riding on his little shoulders and I'm confident he can handle it."
Casting director Sarah Finn said in a statement that Sethi's "natural charisma and instincts" stood out among the other auditions, and that the young actor embodies "the heart, humor, and daring of the character."
"He's warm and accessible, yet also has an intelligence well beyond his years and impressed us all with his ability to hold his own in any situation," Finn added.
Sethi will be the only actor on screen during filming. Talking animals -- including those voiced by Scarlett Johansson, Idris Elba, Lupita Nyong'o, and Ben Kinglsey -- will be computer animated and added in post-production.
"The Jungle Book" is due in theaters on October 9, 2015.