The eight-year-old, who has only been named as Joe, was found alive after surviving for more than a week since he went missing from his family garden in Oldenburg on June 17.
His disappearance sparked a massive search involving local residents, detectives and dog handlers, but no sign of him was found until a passer-by called police reporting a “faint whimpering sound” coming from a sewer on June 25.
Firefighters pulled the boy from the sewer, which was about 300 metres from his home and brought him to hospital where he was discovered to be suffering from hypothermia, but no other major injuries.
Police broke the news by tweeting: "Eight-year-old Joe lives!"
"Thanks to a tip from the public, we were able to find Joe in a sewer system," police chief Johann Kühme said.
"The most important thing is that he is alive and was immediately taken to a hospital where he is in good hands. We can all breathe a sigh of relief."
Police have now ruled out any foul play, as they established the boy crawled into the sewer on the day he went missing.
His father told local media Joe was "doing well" given the circumstances.
An eyewitness told local media: “I saw how the boy was carefully pulled out of the hole, a helper calmed him down.
“I didn't know it was Joe. He looked unharmed. I can't believe he was in there and alive!'
And an unnamed neighbour added: 'A rescuer calmed the child down. He looked unharmed.
“It was very fortunate that he was heard – a lot of cars pass through here and the noise level is higher there.'
Another told German newspaper Bild: “I pass by here often. If he had been there for a long time someone would have heard him earlier.”
The Oldenburg force is now planning to use specialised robots to travel through the drains and try and establish how Joe came to be stuck down there.
Due to the narrowness of the sewer, no adults could pass through, police have confirmed.
Retired emergency doctor Peter Sefrin (81) told local media: “At these temperatures, the heat is an important factor.
“But in the canal, it is naturally cooler. Humans can survive a good three days without water and food. That's no problem.”
He added: 'We know that people who were buried survived in caves from dripping condensation water. Maybe that was the case here too.'