West Ham have signed midfielder Sead Haksabanovic from Swedish club Halmstads BK for an undisclosed fee.
The 18-year-old, who is a Montenegro international, has signed a five-year contract at the London Stadium, the Premier League club announced on their official website.
"I'm really happy to be here - it's a dream come true," said Haksabanovic.
"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."
Haksabanovic, who has had trials with a host of Premier League clubs including both Manchester sides and Chelsea, became the second youngest player in Allsvenskan history when he made his debut in April 2015.
Meanwhile, West Ham have received special dispensation to play their Carabao Cup second-round tie away from home as the London Stadium will still not be ready to host football matches.
The Hammers have already had permission to play their opening three Premier League games on the road as the venue will be altered back into football mode having hosted the athletics World Championships.
The track and field event does not finish until August 13, with seating and signage then requiring changing before West Ham move back into the stadium.
Now the English Football League (EFL) has confirmed Slaven Bilic's side will be also allowed to move their second-round Carabao Cup tie if they are drawn at home - with a compensation payment to be made to their opponents.
"In the event that West Ham United are drawn at home in round two of the Carabao Cup then they will be given special dispensation by the EFL Board to play the fixture at the opposition's venue due to a previously agreed arrangement that the London Stadium was to stage the 2017 IAAF World Championships in August," the statement read.
"If the draw dictates this is required, then the EFL Board will determine a level of compensation to be paid by West Ham United to the opposing club. No further 2017/18 Carabao Cup ties will be affected in the event West Ham United progress in the competition and are drawn at home."