Hong Kong police arrested the former executive editor-in-chief of the now-closed pro-democracy Apple Daily tabloid on Wednesday, local media reported, the latest arrest in a national security investigation of media tycoon Jimmy Lai's newspaper.

Police, who typically do not identify individuals under investigation, said in a statement they had arrested a 51-year-old former male newspaper editor on suspicion of "conspiring to collude with foreign countries or foreign forces to endanger national security."

Media group Next Digital, which published Apple Daily, did not...