Deputy Chief Justice Luke Malaba said the court found that the law was unjustified and "had the effect of interfering with the exercise of the right to freedom of expression."
Zimbabwe has tough media laws which have been used by President Robert Mugabe's government to expel foreign correspondents and arrest journalists from the independent press.
But the authorities are tinkering with hundreds of statutes after the country last year adopted a new constitution which curtails the president's powers, among other provisions.
The journalists, Constantine Chimakure, a former editor at the Zimbabwe Independent and Vincent Kahiya, the group editor-in-chief, challenged the constitutionality of the law, arguing that it was not reasonably justifiable in a democratic society.
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In October last year, Chief Justice Godfrey Chidyausiku said sections of the statutes which provided for criminal defamation had a chilling effect on those seeking to express their right to freedom of expression.
The top judge also summoned Justice Minister Emmerson Mnangagwa to court to give reasons why the criminal defamation law should be retained in the country's statutes.
However, he said he did not intend to oppose the challenge to the law.