The 18-year-old has penned a five-year-deal with the Hammers, joining from Swedish club Halmstads BK, where he became the second-youngest player in Allsvenskan league history in April 2015.
Haksabanovic’s transfer fee is undisclosed, though reported to be around the region of £2.5m.
The teenager has already made 75 first-team appearances with Halmstads, making his international debut earlier this May.
“I’m really happy to be here – it’s a dream come true,” Haksabanovic told the West Ham website.
“West Ham is a big club and it is a nice feeling to have signed. The move happened quite fast – we heard that they wanted me, so I was interested to hear the plan for me and when I did I was very pleased.
“I am an attacking player, I like to dribble and shoot, but I work hard for the team. I am really looking forward to getting started, I have watched quite a few West Ham games in the Premier League and I can see that they have some great players and great fans.
“Now I just want to get used to being with the team and then hopefully play some games at some point next season.”
Welcome to West Ham United, Sead Hakšabanović! ⚒ pic.twitter.com/hYSkUZIatX
— West Ham United (@WestHamUtd) August 7, 2017
Sead Haksabanovic’s career so far – and the stats behind him
An attacking midfielder with relentless speed, Haksabonivic bears impressive technical ability with a keen eye for goal.
Having joined Halmstad at the age of six in 2006, he became the Swedish club’s youngest-ever player as he made his debut at IFK Norrkoping in the Allsvenskan in April 2015, aged 15.
HBK were relegated in 2015, but Haksabanovic’s contribution of eight goals and seven assists in in 30 overall appearances saw the club promoted from the Superettan the following season.
Last season, the teenager continued his strong display of playing form, starting all 18 Allsvenskan games and finding the back of the net four times.
On international level, the teenager featured for Sweden at U17 and U19 level before deciding to play for Montenegro, the country of his father’s birth, earlier this year. He was handed his first senior cap in June, coming on as a substitute for Montenegro’s 4-1 World Cup qualifying win against Armenia.