Translation guide
A police officer who is not in uniform, working undercover or in civilian clothes.
Refer to a police officer who is not wearing a uniform, typically for investigative or undercover work.
Standard term for a plainclothes police officer. Literally 'civilian clothes police officer'.
Plainclothes police officers are patrolling the station.
Specifically a plainclothes detective. '刑事' means detective/investigator.
Plainclothes detectives are on a stakeout.
Descriptive phrase: 'police officer in civilian clothes'. Slightly more formal or explanatory.
A plainclothes police officer tailed the suspect.
Emphasize the covert or undercover nature of the officer's role.
Literally 'masked police officer', but often used for undercover officers who conceal their identity. Can also refer to officers in disguise.
An undercover officer participated in the sting operation.
Undercover investigator, often used for officers infiltrating criminal organizations. More specific to deep cover operations.
私服警官 simply means an officer not in uniform, often for investigative work. 覆面警官 implies a higher level of concealment, sometimes with a disguise or mask, and is more associated with undercover operations.
In Japanese, the specific term used can depend on the context: general plainclothes duty (私服警官), detective work (私服刑事), or deep undercover (潜入捜査官). Choose based on the nuance you need.
An undercover officer has infiltrated the organization.