The 33-year-old moves back to the Weser for a one-year loan, with the two clubs confirming the deal on Tuesday, only two days after it first emerged that Bremen were exploring a fresh return for their academy graduate.
No purchase option was included in the agreement, and Füllkrug is understood to have accepted a reduction in salary to help get the move over the line.
"We're very pleased that we were able to complete Niclas's transfer. Fülle has made a clear commitment to Werder.
"Niclas knows the Bundesliga and is, in any case, an immediate boost for us.
"We know that the last two years haven't gone quite the way he had imagined, but we're convinced he can continue his career successfully at Werder," said Peter Niemeyer, the club's head of professional football.
It marks the second time Füllkrug has returned to his training club, having previously left Werder Bremen for Borussia Dortmund before his move to West Ham in the summer of 2024. The 24-cap Germany international has been searching in vain for the form that earned him call-ups to the 2022 World Cup and Euro 2024 ever since that switch to London.
A series of persistent injuries hampered him first in England and then during a loan spell at AC Milan last season. Füllkrug will now hope life back at Werder helps him rediscover his best form and rebuild his case for a return to the national team, in a city where he remains a firm fan favourite from his previous stints.
Head coach Daniel Thioune was equally pleased to welcome back a player he expects to add immediate quality to his attack. Füllkrug remains under contract at West Ham until 2028, meaning this loan represents a chance for both player and club to reassess his situation without either side making a long-term commitment.
He came through Werder's youth ranks as a boy before making his senior debut for the club, and has since gone on to represent Germany at both the 2022 World Cup and Euro 2024, scoring 14 goals in 24 international appearances along the way.