The 45-year-old king noticed Sindiswa Dlamini during a traditional reed dance last month, which is set up to allow the king a choice for his next wife, the British newspaper The Daily Telegraph reported.
Dlamini graduated from high school in the small country's capital last year.
The king's selection comes at a tense time for the country, where 69 percent of the population lives below the poverty line.
The country also has the world's highest AIDS rate.
The citizens will vote in parliamentary elections Friday, but critics say the votes will change nothing since the king is an absolute monarch and parties are banned, the Telegraph said.
The king uses an estimated $44 million of the country's annual budget to house his 24 children and 14 wives in palaces.
In recent years, three of the king's wives have fled due to emotional and physical abuse.