King Mswati III's right-hand man Timothy Velabo (TV) Mtetwa said it is all right for under-aged girls to be taken into traditional marriages.
This is despite a newly-enacted Children's Protection and Welfare Act, 2012, that aims to make the practice known as kwendzisa illegal.
Mtetwa, who is Ludzidzini Governor and popularly known as the 'traditional prime minister' of Swaziland, said girls aged 15 can marry if their parents agree and the child wants to.
Mtetwa is considered in Swaziland to be the ultimate authority on traditional law and custom in the kingdom. He was responding to a question from the Swazi Observer, the newspaper in effect owned by King Mswati.
He admitted that the law existed which made it illegal to engage in sexual relationships with girls under the age of 18. But, he said in the past kwendzisa was allowed and he was not aware of any order that stated that it was now banned.
'What I know is that if the parents and the girl have agreed, the authorities never penalised anyone,' he told the newspaper.
The Children's Protection and Welfare Act was endorsed by King Mswati in September 2012. The law states that all practices that are likely to affect the child's life, health, welfare, dignity or physical, emotional, psychological, mental and intellectual development are illegal.
Offenders can be fined up to E10,000 (US$1,000).
Although the Act has been passed, it is not clear when it will come into force.
In September, the Times of Swaziland, the only independent daily newspaper in the kingdom reported that Swaziland ranked 45 out of 60 countries, according to the international ratings of children's friendliness and rights protection report.
At the time of enactment of the Act, Mtetwa was quoted by the Times saying traditionalists would apply for a review of the Act if it was felt to collide with Swazi customs and traditions.
There have been a number of controversial cases in Swaziland recently where girls have been forced into having sex. Swaziland's Deputy Prime Minister Themba Masuku criticised men who had sex with girls under the age of 18 after media reports that a soccer star claimed he was in a relationship with a 14-year-old girl after being arrested for allegedly raping her.
In the recent past, the Deputy Prime Minister's office has rescued a number of underage girls, who had to drop out of school because they had been 'forced' into marriage by their parents, the Observer reported.