The 21-year-old Belgian arrived at Stamford Bridge during the January transfer window and was immediately loaned back to former club Genk.
"It's good that the transfer to Bremen is now completed," De Bruyne told the club's official website.
"SV Werder have a big tradition and play in one of the best leagues in Europe with full stadiums all the time.
"I would like to prove successful with my new team-mates."
He made his Chelsea debut in the 4-2 friendly victory over Seattle Sounders during the recent tour to America, but Chelsea on Thursday confirmed he is to continue his development at Werder Bremen.