DYFED-Powys Police officers were called to an incident at an address in Francis Terrace, Carmarthen, at around 9.50am on Wednesday, February 20.
A 31-year-old man was taken to hospital with serious injuries but sadly died later that day.
Enquiries are ongoing to establish the cause of death.
There will be heightened police activity in the area today as part of an investigation to establish what happened.
A 28-year-old woman has been arrested on suspicion of grievous bodily harm. She remains in police custody.
Anyone with information that can help officers with their investigation is asked to report it by phoning 101.
If you are deaf, hard of hearing or speech impaired text the non-emergency number: 07811 311 908.
Alternatively, contact the independent charity Crimestoppers anonymously by calling 0800 555111, or visiting crimestoppers-uk.org. Use reference: DP-20190220-098