Jean-Pierre Amougou Belinga, owner of L'Anecdote media group, "was arrested... at dawn," the company said.
Denis Omgba Bomba, head of the National Media Observatory, a unit attached to the communications ministry, said the tycoon was a "suspect in the killing of Martinez Zogo."
Zogo, who was abducted and brutally murdered last month, was outspoken against graft and financial sleaze and had often faced threats over his work.
The journalist, 50, was the manager of the privately-owned radio station Amplitude FM and host of a daily show called Embouteillage (Traffic Jam).
Amougou Belinga was one of his targets. The tycoon, reputedly a friend of several ministers, has holdings in banking, finance, insurance and property, as well as L'Anecdote, which owns a daily newspaper of that name and several pro-government TV and radio stations.
Just days before he was killed, he had told listeners about threats he faced.
Zogo was abducted on January 17 outside a police station in the suburbs of the capital Yaounde, and his heavily-mutilated corpse was found five days later.
Amougou Belinga's arrest came after several people were detained last week in the affair.
The murder sparked an outcry, including a protest by 20 leading Cameroonians over the government's "long tradition of trivialising impunity and accepting atrocities."
President Paul Biya, who has ruled Cameroon with an iron fist for more than 40 years, called for a full probe into the murder.
The government has insisted Cameroon is "a state of law, where liberty is guaranteed, including the freedom of the press".
RSF's Press Freedom Index ranks Cameroon a lowly 118th out of 180 countries.