The transfer fee is expected to be 9.5 million pounds ($12.45 million) according to British media reports.
"I'm very glad and happy and proud to finally be here and officially become a Fulham player".
"I'm happy to the moon and back", he said.
"The 23-year-old striker proved to be the flawless fit for Fulham, scoring 12 times in just 17 starts to help the team reach Wembley".
Full Brexit deal with European Union by March will be a challenge - Raab
However, he criticised "selective" reports claiming the government was also planning to stockpile food and medical supplies. Precise details will not be known until the withdrawal agreement is concluded.
The 26-year-old, who played for Japan at this summer's World Cup, scored a personal-high eight goals in 27 matches in his third season with the German club.
With no cover for DeAndre Yedlin either (apart from Manquillo...), it only leaves Rafa Benitez with five fit defenders who are up to the job: Lascelles, Clark, Yedlin, Dummett, and Schar - if he proves to be able to handle the Premier League.
At normal clubs, it is usually the Championship club loaning a player off a Premier League one, not the other way around!
"It means we can bring in another striker and we can be stronger", he said.
"He was a hero in our run to promotion, and from the moment he arrived at Motspur and began working with us, we've always wanted Mitro on a permanent; I told him that after the final at Wembley, and I'm thrilled to make it official today".