Electing to bat first, Australia were dismissed for 216 runs in 47.2 overs. Ishan Porel, Shiva Singh and Anukul Roy picked up two wickets each for India.
Opener Manoj Kalra then spearheaded the Indian run chase with an unbeaten century as the Boys in Blue romped to victory with 67 balls to spare.
Kalra remained unbeaten on 101 runs off 102 deliveries while Harvik Desai was not out on 47 at the other end.
Desai brought up the title win in style, smashing a boundary off Australian pacer Will Sutherland.
India thus became the first nation to win the ICC U19 World Cup four times. They had earlier won the title in 2000, 2008 and 2012.
Australia are second on the list with three titles.
This is the second time that India have defeated Australia in the final. They had earlier defeated the boys from Down Under in the final of the 2012 edition.