Haksabanovic was handed his debut for Halmstads in 2015 at the age of 15 and the attacking midfielder has scored four goals in 18 appearances in the Swedish Allsvenskan this season.
He represented Sweden at youth level but chose to switch allegiances to Montenegro and made his debut for the national team in June this year, coming on as a substitute in the 4-1 World Cup qualifying win against Albania.
"I'm really happy to be here - it's a dream come true," Haksabanovic told the club's official 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."
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