Emergency services responded to a property on London Road in Holyhead, Anglesey during the early hours of Saturday morning following reports of a sudden death.
North Wales Police received notification from the Welsh Ambulance Service at 2.42am on October 17 regarding an incident at Llys Watling.
Paramedics attended the location and discovered a man who could not be revived despite the efforts of emergency personnel.
The individual was declared deceased at the scene.
Officers subsequently took a man from the local area into custody in connection with the circumstances surrounding the death.
Authorities have not disclosed the specific nature of the offences under investigation.
Detective Chief Inspector Richard Sidney stated that additional officers would be visible throughout the neighbourhood while investigators conducted door-to-door inquiries and pursued further lines of enquiry.
Local residents were assured that the matter was being treated with priority.
Those possessing details relevant to the case have been requested to reach North Wales Police through the force website or by telephoning 101, citing reference number 26000852942.