Iran Telecommunications Company, which has a monopoly on the country's telecommunications, set out the new rules on Internet use.
Reuters cited the Hambastegi newspaper as saying that ISPs are now obliged to block some sites for reasons of morality and state security. Undesirable sites include the Web sites of opposition groups. Surfers under the age of 18 will also be denied Web access.
Any ISPs which do not observe the rules face court action and the loss of their operating licenses.
Last month, police closed upwards of 400 Internet cafes in the capital city of Tehran, telling the owners they had to get licences. Reuters estimates that Tehran boasts 1,500 Internet cafes.
In a country which imposes strict Islamic laws, and where the media is tightly regulated, the Internet and Internet cafes are popular with the nation's youth.