“Since the animal was already climbing over the outer fence [of the zoo] it was no longer possible to tranquillize it,” a statement from city authorities read.
“Unfortunately, a person authoried to use firearms was forced to use his weapon.”
Dozens of police and firefighters had rushed to the zoo after news of the animals' escape before the all-clear was sounded at 9 pm.
Orang-utans can weigh up to 100 kilgrammes, and are considered dangerous enough that police officers were armed with submachine guns as they searched.
“At the end of the day, the protection of human life had to come first,” city spokeswoman Anja Kopka told Ruhr Nachrichten.
She added that zoo workers had been deeply affected by the orang-utan's death.
The other orang-utan who escaped was tranquillized and brought back to its cage inside the zoo. It was not immediately clear how the escape happened.