A police source told AFP that former executive editor-in-chief Lam Man-chung had been detained.
In a statement, police said they had arrested a 51-year-old former newspaper editor for "collusion with foreign forces", a national security crime.
Lam is the ninth employee of Apple Daily arrested under a sweeping national security law that Beijing imposed on Hong Kong last year after huge and often violent democracy protests.
Apple Daily, an unapologetic backer of the democracy movement, put out its last edition last month after its top leadership was arrested and its assets frozen under the security law.
Lam was the editor who oversaw that final edition, ending the paper's 26-year run.
Authorities said Apple Daily's reporting and editorials backed calls for international sanctions against China, a political stance that has been criminalised by the new security law.
The tabloid's owner Jimmy Lai, 73, is currently in prison and has been charged with collusion alongside two other executives who have been denied bail.
They face up to life in prison if convicted.
Among the others arrested, but currently not charged, are two of the paper's leading editorial writers, including one who was detained at Hong Kong's airport as he tried to leave the city.
The paper's sudden demise was a stark warning to all media outlets on the reach of a new national security law in a city that once billed itself as a beacon of press freedom in the region.
Last week the Hong Kong Journalists Association said media freedoms were "in tatters" as China remoulds the once outspoken business hub in its own authoritarian image.
