Wendy Hleta said police would enforce an 1889 law which bans “immoral” dressing if they receive a complaint.
She also said women in the deeply conservative kingdom make it easier for rapists by wearing mini-skirts.
Last month, police reportedly blocked women in mini-skirts marching against rape in the second city, Manzini.
However, the colonial-era law does not apply to traditional costumes worn by women during ceremonies like the annual reed dance, where the monarch may choose a new wife.
In 2000, the government introduced a law requiring school girls aged 10 years old and above to wear knee-length skirts to curb promiscuity as part of attempts to halt the spread of Aids.
The country has a population of more than 1.2 million and one of the highest HIV/Aids rates in the world.
‘Undressing people with their eyes’
Ms Hleta said the 1889 law had not been enforced recently, but police wanted to alert women about its existence after receiving complaints from some men in Manzini about women wearing mini-skirts.
Anyone arrested and guilty of “immorality” under the Crimes Act of 1889 could receive a fine of up to $10 (£6) or a jail-term of up to six months if they failed to pay the fine, she said.
Women should be careful about wearing revealing clothes, the police spokeswoman said.
“We do not encourage that women should be harmed, but at the same time people should note acceptable conduct of behaviour,” Ms Hleta is quoted by the privately owned Times of Swaziland newspaper as saying.
“The act of the rapist is made easy because it would be easy to remove the half-cloth worn by the women,” she said.
Women who wear “skimpy clothes” also draw unnecessary attention to themselves, Ms Hleta said.
“I have read from the social networks that men and even other women have a tendency of ‘undressing people with their eyes’. That becomes easier when the clothes are hugging or are more revealing,” Ms Hleta is quoted as saying.
However, the law excluded exposure of the body due to breast feeding and wearing cultural regalia, she said.
Swaziland is a patriarchal society, ruled by sub-Saharan Africa’s only absolute monarch, King Mswati III.
He has 13 wives and is often accused of leading a flamboyant lifestyle.
But in a move that was widely welcomed by rights groups, Swaziland’s Ellinah Wamukoya was last month consecrated as the first woman bishop in Africa by the Anglican church.