The suspect, Koral Karsan, lives in Turkey and is currently out of reach of German law enforcement, police said as they presented the late Beatle's rediscovered personal effects at a press conference.
The former chauffeur worked for Ono from 1995 to 2006. He already spent 60 days in jail in 2007 in the United States for trying to blackmail Lennon's widow Ono.
“We believe (he) is guilty and the suspected thief of the objects,” Carsten Pfohl of Berlin's criminal police office told reporters.
The possessions were stolen from Ono's New York home in 2006.
A total of 86 items – including postcards, letters, handwritten music scores and two pairs of Lennon's trademark round glasses – were discovered at a bankrupt Berlin auction house in July.
More items were found hidden in the car of another suspect in the case, a 58-year-old German national who was arrested Monday on suspicion of fraud and handling stolen goods.
One of the three diaries from the haul contains an entry penned by Lennon on December 8, 1980 – the day he was shot dead by a disturbed fan outside his Manhattan apartment building.
“We were amazed by what we found,” said Berlin prosecutor Michael von Hagen, calling the trove part of “music history”.
Along with co-Beatle Paul McCartney, Lennon wrote some of the Fab Four's biggest hits including “Help” and “With a Little Help from My Friends”, before the band split in 1970.
His possessions have since become collectors' items.
A leather jacket supposedly worn by Lennon sold for £10,400 (€11,700; $13,800) at an auction in England in February.