The 78-year-old is in critical condition after sustaining massive head and chest wounds from the weight of the 100-kilogramme tree at a shopping centre in the city's Wandsbek district.
The five-metre Tannenbaum had been attached to the ceiling with three steel cables, and the centre is not sure why it came loose, a police spokesperson said.
After the accident other shoppers helped treat the woman until emergency workers transported her to a nearby hospital. Her injuries are reportedly life-threatening.
Police are now investigating the firm employed by the store for Christmas decoration for negligent bodily injury, the spokesperson said. The TÜV safety certification organisation has also been contacted to review the case.