The 32-year-old man was arrested during the early hours of Monday morning after a woman was admitted to Pinderfields Hospital with serious head injuries after being found at a property on Smawthorne Grove in Castleford.
Police were called to the hospital at around 5.30am.
The woman was later transferred to Leeds General Infirmary for continuing treatment but died from her injuries later on Monday morning.
Police have confirmed the man remains in custody under arrest and detectives are continuing their investigation.
Detective Chief Inspector Emma Winfield of the Homicide and Major Enquiry Team, said: “Officers are investigating the circumstances of how the victim came to be injured at the property and have now launched a murder enquiry.
“A 32-year-old man remain under arrest in custody for questioning and specially trained officers are working to support the victim’s family.”