A police official said those being held included two convicts serving life sentences, who are thought to have organised the kidnapping from their jail cells, and their wives who are suspected of keeping 74-year-old Pericles Panagopoulos in captivity.
The suspects will be charged later on Friday or on Saturday, the official told Reuters.
Some of those detained are also suspected of being involved in contract killings and the shooting of a well-known night club singer earlier this year in Athens, the official added.
Panagopoulos, one of Greece's richest men and a key player in the country's ferry business, was snatched in broad daylight on Jan. 12 by three men wielding assault rifles as he was being driven to work near Athens.
He was released early on Jan. 20 after the kidnappers called his wife to reveal his location.
Businessmen are often kidnapped in Greece. In most cases, the victims are released after paying a ransom and the kidnappers are arrested after lengthy investigations.