Coach Marc Wilmots had been expected to name a preliminary group of 26 ahead of the tournament beginning in Brazil next month, but surprisingly made a final selection with the only uncertainty concerning the fitness of the third goalkeeper.
"I've gone for 24. I thought it was better to be direct from the start," Wilmots told a news conference.
Januzaj, born and brought up in Brussels by a family with Balkan origins, only opted to play for Belgium three weeks ago after a strong first season in the Premier League.
He was also eligible to play for Kosovo, Serbia and Turkey and was even talked of as a potential future England player.
Wilmots described Januzaj as a top talent already used to playing big matches in front of large crowds.
"He can play in various positions. For Manchester United he was the best," he said. "He gives us more options in attack. It's the right moment to bring him in."
The only uncertainty concerns the fitness of reserve keeper Koen Casteels, who broke his shin in early April.
Wilmots has given him the chance to show he has fully recovered by the start of June. Otherwise, his spot will go to Anderlecht's Silvio Proto.
Januzaj will be among 11 squad members who played in the Premier League this season. There are three each from the Belgian and German leagues and two each from Spain and Russia.
The other teenager in the squad is surprise call-up Divock Origi, who scored five goals from 29 appearances with Lille in Ligue 1. Wilmots said the 19-year-old of Kenyan descent bore similarities to Aston Villa forward Christian Benteke, who was ruled out with a torn Achilles tendon.
"We watched him against Marseille, Bordeaux and Paris," Wilmots said. "He is very quick, has fine technique and a good attitude."
The oldest player, 36-year-old centre back Daniel Van Buyten, is the only squad member with experience at a major tournament, featuring in the 2002 World Cup alongside Wilmots.
Belgium, appearing in the finals for the first time since 2002, will hold a training camp near the eastern city of Genk from May 19, concluding with a friendly against Luxembourg on May 26.
They then move to Stockholm, with a match against Sweden planned for June 1. After a few days at the Belgian coast, they have a friendly in Brussels against Tunisia on June 7.
They fly on June 10 to Sao Paolo, where they plan a match behind closed doors against the United States two days later. Belgium will face Algeria, Russia and South Korea in Group H.
Goalkeepers: Thibaut Courtois (Atletico Madrid, on loan from Chelsea), Simon Mignolet (Liverpool), Koen Casteels (Hoffenheim), Silvio Proto (Anderlecht).
Defenders: Toby Alderweireld (Atletico Madrid), Laurent Ciman (Standard Liege), Nicolas Lombaerts (Zenit St Petersburg), Vincent Kompany (Manchester City), Daniel Van Buyten (Bayern Munich), Anthony Vanden Borre (Anderlecht), Thomas Vermaelen (Arsenal), Jan Vertonghen (Tottenham).
Midfielders: Nacer Chadli, Mousa Dembele (both Tottenham), Steven Defour (Porto), Kevin De Bruyne (Wolfsburg), Marouane Fellaini, Adnan Januzaj (both Manchester United), Eden Hazard (Chelsea), Kevin Mirallas (Everton), Divock Origi (Lille), Axel Witsel (Zenit St Petersburg).
Forwards: Romelu Lukaku (Everton, on loan from Chelsea), Dries Mertens (Napoli).
Standby: Sebastien Pocognoli (Hannover), Radia Nainggolan (Roma), Thorgan Hazard (Zulte Waregem, on loan from Chelsea), Guillaume Gillet (Anderlecht), Michy Batshuayi, Jelle Van Damme (both Standard Liege).
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