Mr Letherby was taken to the Royal Glamorgan Hospital in Llantrisant, where he later died, after an incident in De Winton Street, Tonypandy late on Friday.
Four people who were arrested in connection with his death remain in custody.
South Wales Police has urged anyone with information to call 101.
A man who died after a serious assault in Rhondda Cynon Taff has been named as 42-year-old Wayne Letherby.