Police were called to a house in Taff's Well at about 11:45 BST on Monday and arrested a 22-year-old-man.
South Wales Police said there were a large number of officers in the area investigating and it is not looking for anyone else.
An appeal has been made for witnesses.
A woman in her 50s is in hospital with potentially life-threatening injuries after an incident in Rhondda Cynon Taff.