The absolute monarchy, which imposes strict Internet censorship, banned the service in June after Viber failed to comply with unspecified regulations.
"The service is back with no problems," said Khaled Mohammed, also after installing a Viber update.
But the Communications and Information Technology Commission said the ban remains in place.
"There may be some illegal way of providing the application by the Viber company to get round the ban, but the commission is dealing with that," said CITC spokesman Sultan al-Malek.
"The CITC has not lifted the ban as the company has not complied with the requirements of the commission."
Authorities had asked telecommunications providers to provide a mechanism that would allow censorship, industry sources said when the ban was first imposed.
In March, service providers were told to work with the developers of such applications to "quickly meet the regulatory conditions".
The commission warned of "suitable measures" if providers of messenger apps WhatsApp, Skype and Viber failed to comply with its rules.
(Also see: WhatsApp could be banned in Saudi Arabia)
In 2010, the conservative kingdom and neighbouring United Arab Emirates threatened to ban BlackBerry instant messaging and demanded that it instal local servers allowing the service to be censored.
BlackBerry instant messaging services remain uninterrupted, but it is not clear how far the Canadian smartphone manufacturer went to comply.