Contacted by AFP, a police spokesman in the southwestern city declined to give details about the hostage or the possible motivation behind the incident.
“At 8:20 p.m. (1920 GMT) the male and armed perpetrator left the building with a hostage and fled,” Ulm police said in a statement.
Police fired shots and apprehended the suspect, while the hostage remained unharmed, authorities said.
The hostage situation began around 6:45 pm, police said.
An area around the square and the city’s main landmark, the Ulm Minster church, were subsequently closed off.
According to reports by local media, the hostage situation took place in a Starbucks franchise.
Last year, the Hamburg airport was closed after a man took his four-year-old daughter hostage and barricaded himself in his car on the tarmac.