Video footage posted on social media shows one officer pinning the boy's hands behind his back and the other kneeling on his neck, SkyNews reported.
Onlookers can be heard saying "that's not funny" and "move your knee", according to local media.
It comes after worldwide protests sparked by the death of George Floyd, an unarmed black man in the US who died after police officers knelt on his neck despite his cries of "I can't breathe".
The German incident, which happened at around 7.30pm in Dusseldorf's Old Town on 15 August, has now been given to a nearby force to investigate, reports Euronews.
The four Dusseldorf Police officers involved are being probed by Duisburg Police for "reasons of neutrality", the force said in a statement.
Officers were originally called to a restaurant in Dusseldorf to reports of 10 people rioting, German police said.
The boy involved is believed to have resisted arrest and later assaulted officers, according to police.
He was taken to a police station for questioning but was then handed back to his legal guardians.