The 18-year-old, who cost a reported £2.7m, has agreed a five-year contract.
Haksabanovic is the Swedish top flight’s youngest ever debutant, with the Montenegro born star making his debut for Halmstads at the age of only 15. He has since made over 70 first team appearances, and made his international debut earlier this year in May.
“I’m really happy to be here – it’s a dream come true,” said Haksabanovic, who follows in the footsteps of fellow former Halmstads star Fredrik Ljungberg.
“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.”