The Cottagers announced the signing of the former Tottenham right-back on Monday.
The 22-year-old had only moved to City on a permanent deal earlier this month and the Robins reported Fredericks wanted to return to the capital “for personal reasons”.
Fredericks told Fulham’s official website: “When I heard of the interest from Fulham I jumped at the chance and I’m delighted that we got it done so quickly.
“It’s a massive club that belongs back in the Premier League. Fulham’s a great area with big fans – it’s a good club.”