According to daily Berliner Zeitung, the suspect is the 28-year-old uncle of one of the other suspects already in custody.
Investigators believe he helped plan the armed robbery and drove the getaway vehicle parked outside Berlin's luxury Park Hyatt hotel where the tournament was underway.
The man, who is reportedly of Lebanese background, was arrested on Sunday evening.
Four masked men armed with machetes managed to swipe some €240,000 in buy-in money from the tournament on March 6. Police are still looking for the money.
The four robbers are in police custody after another two suspects were arrested on Saturday.
The arrests concluded a sweeping international search effort to track the suspects with names and photos. Police chief Dieter Glietsch said early on that he was certain the four thieves would be arrested quickly, but a full week passed before investigators achieved key breaks in the case.