35-year-old Kripal Singh was shot at by two persons who came on a motorcycle in the Puwayan area in Shahjahanpur when he was on his way home from the market, police said.
Before escaping, the duo warned him against deposing against Asaram.
Mr Singh, who sustained serious bullet injuries, was rushed from Shahjahanpur to a Bareilly hospital where he succumbed to injuries at around 11:25 pm, Bareilly Circle Officer Mukul Dwivedi said.
Mr Singh's body was being taken back to Shahjahanpur for cremation, he said.
Special police teams have been constituted to track down the attackers, they said.
In a statement given to Additional City Magistrate, Mr Singh alleged that people connected to Asaram were threatening him for the last few days.
Police said Mr Singh was an employee of a transport firm whose owner's daughter had lodged a case against 74-year-old Asaram for allegedly sexually assaulting her at his ashram near Jodhpur in Rajasthan in 2013.
Mr Singh was a key witness and his statement in the case against Asaram had been recorded around three months ago.
Asaram has been in a jail in Jodhpur since September 2013. Two months later, he, along with his son Narayan Sai, was booked for the alleged rape of two sisters at their ashram in Gujarat's Surat.
In the last attack against Asaram's witness, his cook was shot dead in Muzaffarnagar in Uttar Pradesh in January.
Nine witnesses have been attacked so far and two have died.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.