Bremen have no option to make the move permanent with De Bruyne highly regarded at Stamford Bridge.
"We have Kevin for several months," Allofs told the club's official website. "We are glad that we can increase the quality in our squad with this commitment."
De Bruyne was happy to make the move to Bremen for next season, saying: "It's great the move to Bremen is done.
"Werder has a great tradition, playing in one of the best leagues in Europe with a full stadium and I want, with my new team-mates, successful football."