The attacker moves to The Tigers in a deal worth £21 million, penning a bumper five-year-deal.
Hull have now spent over £100 million in the transfer window.
Ansah told Hull City's club website: "I’m very excited.
"It’s a dream come true for me to play in the Premier League, and for such a big club – it’s really an honour for me.
“I have heard a lot about the club, the players and the team, and I’m excited to play with the boys and to get to know them.
“It was an honour and a dream playing in the Bundesliga. Now, I have another dream to play in the Premier League, and I’m going to work hard for it."
The German notched five goals for Union Berlin last season and provided one assist, having come through the ranks at Bundesliga 2 side Padderborn.