Police watchdogs are investigating the response to an alleged visit by a member of the public to Pontypridd police station four years ago to give details about the former singer.
Watkins was jailed for 29 years for serious child sex offences in 2013.
Two other officers from the force are already being investigated.
Three others from South Yorkshire are also the focus of an inquiry by the Independent Police Complaints Commission (IPCC) about the way they dealt with complaints about Watkins.
In relation to the latest case, the IPCC said: "One officer has been served with a misconduct notice by the IPCC to advise that their conduct is being investigated."
A third officer from South Wales Police is being investigated in relation to the force's handling of complaints about paedophile Ian Watkins.