About 1,000 athletes, aged between 14 and 18, will participate in the event, scheduled for 27 July to 1 August.
Boxers Carl Frampton and Michael Conlan, and swimmer Sycerika McMahon are among Northern Irish athletes to have previously competed at the Games.
NI Commonwealth Games chairman Robert McVeigh described it as a hugely significant day for local sport.
"This is the result of many years of lobbying and hard work," said McVeigh.
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"It offers a great opportunity for children to strive towards being part of the largest ever Team NI at a Commonwealth Youth Games."
More than 60 nations were represented at the last Youth Games, staged in Samoa in September, with Northern Ireland finishing eighth in the medal table.
The next Youth Games will take place in the Bahamas in 2017, after which it will revert to a four-year cycle.
The formal announcement of Northern Ireland's hosting of the 2021 Games will take place at Belfast City Hall on Monday, with Commonwealth Games NI patron Dame Mary Peters in attendance.
It is expected the swimming competition will take place at the Aurora complex in Bangor, with the Mary Peters Track in Belfast hosting the athletics, and the gymnastics being held in Lisburn.
Northern Ireland were the only bidders for the 2021 Games, after Jersey and Botswana withdrew.
Northern Ireland has been chosen to host the Commonwealth Youth Games in the summer of 2021.