The French striker has completed a move from Eintracht Frankfurt to West Ham in a record deal for the Premier League club.
"I feel really great," Haller told West Ham's website. "I think it's a really good opportunity for me to be here.
"It means a lot [to be the record signing]. It proves that the club [has] really wanted me for a long time. I felt this desire to sign me and I'm really happy to sign, also. It is an ambitious club, a nice club, a nice city and nice fans too."
Haller scored 20 goals in all competitions in 2018-19, including five in Eintracht's run to the semi-finals of the Europa League.
He will be tasked with spearheading the West Ham attacking following Marko Arnautovic's move to Chinese Super League side Shanghai SIPG.
Manuel Pellegrini's side had been tipped to sign Maxi Gómez before turning their attentions to Haller after the Celta Vigo forward moved to Valencia.
"I will always give 100 per cent and respect the club," Haller added.
"I think this is a normal thing and that I can guarantee. I know it's a big transfer and there is a lot of expectation around me, and I just want to make people proud of this transfer and I will give everything I have."