The 29-year-old, who has won four international caps, joins on a four-year contract subject to international clearance and a work permit.
He kept 13 clean sheets in 34 matches for Freiburg this season, the most in the Bundesliga.
"I'm a ball-playing goalkeeper. I try to bring calmness into the squad," said Flekken.
"I'll always try to direct the guys in front of me - I'll help them to help me.
"If you had told me seven years ago when I was in the third division of Germany that, one day, I would be playing in the Premier League, I would have probably knocked you down.