Defamatory statements in print and broadcast media were already punishable by up to two years in prison.
The new legislation has been criticized by some media rights groups and Tajik journalists, who fear it represents an effort to hinder the free flow of information.
Critics predict government officials will try to use the new legislation to punish reporters who criticize them or their activities.
But the head of the Tajik Center for Media Studies takes a different view. Khurshed Atoullo says authorities will find it difficult to prosecute their toughest critics.
"I think it is simply impossible to implement this law," Atoullo says. "Because those websites who publish critical materials about Tajikistan -- or use more or less libel and defamation in their articles -- usually have a domain outside Tajikistan."
Nuriddin Qarshiboev, who heads Tajikistan's National Association for Independent Media, told RFE/RL predicted that that the new law will increase self-censorship among Tajik journalists.