The 12-metre giant was seen on Friday evening near the Baltic Sea island of Rügen, the German Oceanographic Museum in Stralsund reported late on Tuesday.
An ornithologist sighted the whale and followed it for some two hours, the museum said.
Museum whale researcher Harald Benke called the sighting a sensation. The last whale sighted in the area in the 1970s was 10-metres long and earned the nickname “Ossi.”