The trial allows people who have signed up for a special card to use Portslade and Woodingdean libraries.
A phone line to a security company will be available in case of emergency and library staff can be contacted via a dedicated helpline.
Brighton and Hove City Council said staffing will not be reduced.
The libraries are normally closed on Tuesday, Wednesday and Thursday.
Alan Robins, deputy chairman of the council's culture committee, said if the pilot was successful it could be rolled out across the city.
"It's monitored by CCTV. If you go in using your card people are watching you.
"You couldn't just walk out with a load of computer equipment."
Two libraries in Brighton are being kept open for longer hours without any staff under a pilot scheme in the city.