Police said the woman was arrested and she is believed to have acted alone. There was no immediate information on a possible motive.
The attacker targeted people on the platform between tracks 13 and 14 in the station, according to police.
The station in central Hamburg, Germany's second-largest city, is a major hub for local, regional and long-distance trains.
Hamburg's fire service said six people sustained life-threatening injuries, while another three were seriously injured and three had slight injuries, German news agency dpa reported.
Police, however, said on social platform X that there were no "valid figures" yet but that "several" people had life-threatening injuries.
Regional public broadcaster NDR reported that the attack took place shortly after 6pm in front of a waiting train.
A high-speed ICE train with its doors open could be seen at the platform after the attack.
Railway operator Deutsche Bahn said it was "deeply shocked" by the attack. Four tracks at the station were closed Friday evening and some long-distance trains were delayed or diverted.
Carrying weapons, including knives, is banned at the station and on local transport in Hamburg.
