Hertha Berlin sign Piatek from AC Milan

AC Milan's Krzysztof Piatek (R) has signed with German Bundesliga side Hertha Berlin

Hertha Berlin have signed Poland international forward Krzysztof Piatek from AC Milan for a reported 23 million euros ($A38 million).

Piatek, 24, has agreed long-term contract, the Bundesliga club said on Thursday without giving further details.

Sport director Michael Preetz said Piatek had shown his attacking quality with Milan and Poland.

"He now gives us even more quality in attack," he said.

"I am sure that he will help us a lot with the further development of Hertha BSC."

Piatek has played in Italy since joining Genoa in July 2018, scoring 19 goals in 21 games. He moved to Milan in January 2019 and notched 16 goals in 41 matches in league and cup.

"I am very happy that this transfer worked. I feel great desire to help the club together with my new team-mates and to climb up a few places in the table as quickly as possible."

He is the third major signing by Hertha in the winter transfer period, which closes Friday, taking the club's spending to above 50 million euros.

Defensive midfielder Santiago Ascacibar has joined from second division VfB Stuttgart, while midfielder Lucas Tousart has been signed from Olympique Lyon for a reported 24 million euros, a club record, but loaned back to the French club for the rest of the season.

Hertha, lying 13th, play sixth-placed Schalke at home on Friday.