Haller put pen to paper on a five-year contract and becomes the most expensive arrival in club history, as West Ham reportedly paid a fee that could eventually reach £45 million (€50 million), according to the Guardian's Jacob Steinberg.
"I feel really great," Haller said. "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.
"For me, I can guarantee that I will give 100 percent for the club because the club has given me this confidence and I will try to give it back every day in every game."
It's the third time in less than a year that West Ham have broken their transfer record following lucrative deals to sign Issa Diop (£22 million) and Felipe Anderson (£36 million).
The 25-year-old helped Eintracht Frankfurt win the DFB Pokal for the first time in 30 years and scored 33 goals in 77 appearances in the Bundesliga.