20 April 2018 10:05 pm
The monarch has announced the official change in a stadium during celebrations for the 50th anniversary of Swazi independence. The celebrations also marked the king's 50th birthday.
The new name, eSwatini, means "land of the Swazis". The change was unexpected, but King Mswati has been referring to Swaziland for years as eSwatini.
It was the name the king used when he addressed the UN general assembly in 2017 and at the state opening of the country's parliament in 2014.
He explained that the name had caused some confusion, saying: "Whenever we go abroad, people refer to us as Switzerland."
The BBC's Nomsa Maseko in Swaziland has said the announcement of the name change has angered some in the country, who believe the king should focus more on the nation's sluggish economy.
Swaziland's leadership has been criticised by human rights activists for banning political parties and discriminating against women.