Mönchengladbach police spokesman Willy Theveßen confirmed late on Wednesday that a man was in custody, but declined to give further details.
It remains unclear whether police believe the man may have been Mirco's abductor.
“What will become of it, I don't know,” Theveßen said, citing several arrests in recent months that had not led to prosecution in the case.
Krefeld state prosecutors also refused to reveal details of the arrest.
On Thursday morning another police spokesperson said the suspect would be interrogated during the day to check their suspicions. A warrant has not yet been served.
According to daily Rheinische Post the suspect is a father of two in his mid-40s from Viersen. He is allegedly connected to a vehicle sought by police, the paper said.
The boy was last seen September 3 at a bus stop near a skate park in Grefrath, North Rhine-Westphalia, from where he was supposed to ride his bike home. The bike was found by the side of a nearby road.
Mirco's trousers were found in a field in the area, leading police to fear he was the victim of a sexual attack. They also believe the perpetrator lived locally.
Despite several large search actions police have been unable to find the missing boy.
In October detectives said they believed they had identified the make of car the perpetrator drove - a VW Passat Kombi B6 built between 2005 and 2010 - and were making contact with owners of this type of car in the area.
The conclusion was reached by corroborating various witness accounts, though the colour of the car has not been ascertained because it was dark at the time of Mirco's disappearance.