"The Islamic centre of Tajikistan has passed a new special fatwa according to which criticism of the ruling powers will be judged a 'great sin'," said chief mufti Saidmukarram Abdulkodirzoda.
"Criticism undermines trust in the authorities," he warned.
The chief mufti issued the fatwa or religious edict during his Friday sermon at the main mosque in the capital Dushanbe.
Tajik religious experts said the fatwa was agreed with the Central Asian country's authorities and that most moderate Muslims would not obey it.
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"Now it's dangerous to tell the truth. What are we coming to?" he told AFP.
The chief mufti also warned Muslims against making contact with international organisations or media, which he said would "threaten stability in the country and the world."
The Tajik authorities are alarmed at what they see as the radicalisation of young Muslim men amid widespread unemployment in the poorest ex-Soviet country.
Last year around 100 people were arrested, of whom six were jailed, after taking part in training abroad in radical groups. In the last three months alone, 10 Tajiks have been killed fighting on the side of the opposition in Syria.
Tajikistan has been led since 1992 by President Emomali Rakhmon, who came to power amid the chaos of a civil war between the Moscow-backed communist government and the Islamic opposition.