The 27-year-old, who helped Roma finish second in Serie A last season, joined German club for five seasons.
Moroccan international had been a target for Arsenal, Manchester United and Manchester City earlier in transfer window, but the the Bundesliga giant has won the race to sign highly-rated defender from Roma.
“FC Bayern is the best club in the world. I’m delighted that I’m here,” said Benatia cited by the German club website.
“It’s a huge opportunity for me. I’m now playing for one of the biggest clubs in the world,” the defender continued.
“We’re very happy Mehdi has decided to join us. We’re totally convinced by him,” commented FCB sporting director Matthias Sammer.
“He’s a robust, hard-tackling defender. He fills all the criteria required to play for Bayern Munich,” he added.