Another channel of information closed to millions
The only exception is one authorised government office, which a new decree says will be supervised by a reliable person.
Other departments will have to obtain special permission to use the net, from Taleban headquarters in the southern city of Kandahar.
The decree follows earlier moves to deny net access to Afghans.
Read out on the Taleban's Kabul radio station on Saturday, it promised "the necessary shariah punishment" to offenders.
The ban joins a long list of the Islamic movement's prohibitions which already cover television, music and cinema.
Afghanistan's single log-on
The decree was broadcast on state radio.
No reason was given for the ban, which was to be enforced by the Ministry for the Promotion of Virtue and Prevention of Vice.
Our correspondent says it may be that the Taleban feel use of the internet will inevitably lead to other offences, for example those banning images of living beings.
Or that the net throws too many temptations in the path of a believer, particularly the access it gives to pornography.
Economic ban
The Taleban and their supporters do have their own website, but that's PR, intended for external consumption. Officials also surf the net, for example to check out journalists or reporting on their movement.
But like many other non-elected governments they fear contact between their citizens and the outside world.
However almost all Afghans are already banned from access to the internet by economics.
Most people do not have electricity let alone a phone or a computer.
And the country as a whole has only a tiny number of lines, which are linked to the outside system.
Only those citizens lucky enough to live in opposition held Faislabad could soon be connected.
Afghanistan's first internet cafe is due to open there soon.