The 18-year-old has joined the Hammers on a five-year contract for an undisclosed fee from Swedish club Halmstads BK.
He becomes Slaven Bilic's fifth signing of the summer so far after the arrivals of Pablo Zabaleta, Joe Hart, Marko Arnautovic and Javier Hernandez.
And just days into his West Ham career Haksabanovic has already been causing a stir over how to pronounce his name.
Comedian and Gunners fan Whitehall took to Twitter to dish out some banter to one of his old heroes Merson, who is famed for mispronouncing names in his role as a Sky Sports pundit.
"West Ham sign Sead Haksabanovic. Cannot wait to hear Paul Merson trying to pronounce this on Soccer Saturday," Whitehall posted on Twitter.
West Ham sign Sead Hakšabanović. Cannot wait to hear @PaulMerse trying to pronounce this on Soccer Saturday.— Jack Whitehall (@jackwhitehall) August 8, 2017
Haksabanovic has already been taken under the wing of West Ham's record signing Marko Arnautovic.
The pair have been pictured together on social media enjoying one another's company.
West Ham kick off the new season away to Manchester United on Sunday and Merson has backed the Hammers to finish mid table, once place better than last season in 10th.