The 33-year-old has endured a torrid time in east London since joining from Borussia Dortmund in 2024, culminating in West Ham's relegation from the Premier League last term.
Fullkrug's contract at West Ham runs until 2028 but the Hammers did not assign him a squad number this season as they look to bounce back to the Premier League.
Fullkrug scored three goals in 29 matches during an injury-hit spell at West Ham and spent the second half of last season on loan at AC Milan, where he scored once in 20 matches.
Youth product Fuellkrug boasts a better record at Bremen, scoring 49 goals in 124 matches and he helped the club return to the top flight in 2022.
"I've never made a secret of the fact that Werder is a very special club for me. Therefore, I was also prepared to accept a pay cut to make the loan happen," Fu said in a statement.
"It was important to me to play for Werder again."
Fullkrug has scored 14 goals in 24 matches for Germany but has not played at international level since 2025.