The Croatian starlet will join from Dinamo Zagreb and has agreed a five-year contract at Camp Nou, the Spanish champion has confirmed.
The attacking midfielder has long been on the radar of a number of major European clubs, with Manchester City among those credited with an interest in the past, before Barca stepped up its interest in the player earlier this season after sending a delegation to Croatia to thrash out a deal.
The reigning Liga champion has now finally won the race for the 17-year-old's signature, agreeing a five-year contract with the Dinamo academy product for an initial fee of 2.2 million euros, which could rise based on incentives.
"It's difficult to find the words to describe how I feel," Halilovic told Barca's official website. "Many thanks to Dinamo, who will always remain with me as my first love and as the club where I first began to realize my dreams.
"I want to thank all the coaches at the Dinamo academy, and all the supporters. Of course, Barcelona is something special. It's the club I've loved since I was a boy and I feel honored to have the opportunity to play for them.
"First of all, I want to win the double with Dinamo, and then concern myself with my new club."
Dubbed the 'Balkan Messi' by some supporters, Halilovic has established a reputation as one of the continent's brightest prospects since making his debut as a 16-year-old in 2012.
He became the Croatian top flight's youngest-ever goalscorer later that autumn, before making his first appearance in the Champions League against Porto.
Halilovic, who also became Croatia's youngest-ever debutante in 2013, was previously subject to reported interest from Manchester United, Real Madrid and Tottenham, as well as City and Barcelona.