The now 24-year-old was once regarded as one of European football's brightest prospects and LaLiga giant Barca snapped him up from Dinamo Zagreb in 2014.
However, he spent most of his time with Barca's B side and was shipped on loan to Sporting Gijon before joining Hamburg two years after his arrival at Camp Nou.
The move to Germany did not work out particularly well for Halilovic but, after spending a season with Las Palmas on loan, he was signed by Serie A side Milan a couple of years ago.
Loan spells at Standard Liege and Heerenveen did little to convince the Rossoneri, though, and he was released in October.
Halilovic is now set to embark on a new venture with Championship side Birmingham in English football's second tier.
"Honoured and extremely happy to be here! Looking forward to getting started! Keep Right On!" Halilovic posted on Twitter.
Announcing the signing on their official website, Birmingham said: "[The] Blues have completed the signing of highly rated Croatian international, Alen Halilovic, on a short-term contract.
"The attacking midfielder joins until June 2021 having left Italian giants, AC Milan, when his contract expired last month.
"I spoke with the coach a lot and he gave me the confidence, I am now 24 and I really think now is my time to show everyone what I can do." @AlenHalilovic 💪🔵— Birmingham City FC (@BCFC) November 24, 2020
"Halilovic will link up with fellow countryman Ivan Sunjic, having been touted as one of Croatia's most promising stars.
"At just 24 years old, the talented technician has already been snapped up by the likes of Barcelona and AC Milan, and will join Blues aiming to fulfil his undoubted potential."