People who refuse a marriage proposal in Mangalmé in north-eastern Chad must now pay a fine, known as "amchilini".
The region's Higher Islamic Council ruled that the figure be between $23 (£18) and $39 for women and $15 for men.
The Chadian Women's Rights League has launched the hashtag #StopAmchilini to denounce the decision, saying it is illegal as it violates a national law guaranteeing freedom of consent to marriage.
The Islamic Council says its decision was inspired by the Quran.
Forced marriages are prevalent in Chad, with young under-age girls the usual victims.
Despite a law passed in 2015 that prohibits child marriages, 60% of women aged between 20 and 24 were married when they were still children, according to figures from Chad's Institute of Statistics.