According to police, the woman was arrested at the airport in the capital Kuala Lumpur where Kim Jong-nam was targeted in an apparent poisoning on February 13.
The woman was in possession of a Vietnamese travel document.
Malaysian police say they are looking for “a few” other suspects.
According to police, the arrested suspect, who was alone, was identified from CCTV footage taken at the airport. She has been identified as 28 year-old Doan Thi Huong.
South Korean media have widely reported that two women, said to be North Korean agents, were involved and fled the airport in a taxi, though Malaysian police have not confirmed those details.
A grainy image broadcast in South Korea and Malaysia shows a woman wearing a white T-shirt with the letters “LOL” written on the front.
Malaysia is yet to formally confirm that the dead man is Kim Jong-nam, as he was travelling under a different name – Kim Chol. However, the South Korean government has said it is certain it is him.
The South Korean spy agency is said to have told lawmakers they believe Kim Jong-nam was poisoned.
Earlier, Malaysia state news agency Bernama reported that a woman from Myanmar was detained at the airport. It is unclear if that report was referring to the woman now under arrest.
If confirmed, it would be the most high-profile death linked to North Korea since Kim Jong-un’s uncle, Chang Song-thaek, was executed in 2013.
North Korea has not commented on the death but officials from the country’s Malaysian embassy have been visiting the hospital in Kuala Lumpur where Kim Jong-nam’s body has been taken.
On February 13, Kim Jong-nam was attacked while waiting at the budget terminal of Kuala Lumpur International Airport for a 10:00 flight to Macau, Malaysian newspaper reports say, quoting police.
Exactly how the attack unfolded is still unclear. Officials and witnesses have variously said he was splashed with a chemical or had a cloth placed over his face. Earlier reports spoke of a “spray” being used or a needle.
Kim Jong-nam died on the way to hospital.
It was not the first time Kim Jong-nam had traveled under an assumed identity: he was caught trying to enter Japan using a false passport in 2001. He told officials he had been planning to visit Tokyo Disneyland.
He was reportedly targeted for assassination in the past. A North Korean spy jailed by South Korea in 2012 is said to have admitted trying to organize a hit-and-run accident targeting him.
North Korea has a long history of sending agents overseas to carry out assassinations, attacks and kidnappings.
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