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Haller agreed a five-year contract with the Hammers for a reported £45 million (50 millions euros), in a deal that surpasses the £42 million they paid to sign Felipe Anderson from Lazio last year.
The 25-year-old scored 20 goals in 41 appearances for Bundesliga club Eintracht last season.
"West Ham United is delighted to announce the arrival of prolific French striker Sebastian Haller, the club's new record signing," a statement read.
A former France Under-21 international who came through the ranks at Auxerre, Haller will replace Marko Arnautovic at the London Stadium after West Ham sold the Austrian forward to Chinese club Shanghai SIPG recently.
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"I feel really great. I think it's a really good opportunity for me to be here. It means a lot (to be record signing). It proves that the club really wanted me for a long time," Haller told West Ham's website.
"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 per cent for the club because the club has given me this confidence and I will try to give it back every day in every game."
Haller played 75 times for Frankfurt during his two seasons at the club, amassing 32 goals.
He also enjoyed a fruitful spell at FC Utrecht, where he found the back of the net 51 times in 98 games for the Dutch side before moving to Frankfurt.
Haller joins Spain midfielder Pablo Fornals and goalkeepers Roberto and David Martin in signing for West Ham since the end of last season.
He will not link up with Manuel Pellegrini's side until they return from China, where they are currently competing in the Premier League Asia Trophy.