Malaysian police said the man detained on Thursday is the boyfriend of the second female suspect arrested earlier in the day.
Malaysian police on Thursday said the third suspect has been detained in connection with the death of the estranged older half-brother of North Korean leader Kim Jong-un.
"He was detained to facilitate investigations as he is the boyfriend of the second suspect," Selangor state police chief Abu Samah Mat said.
Earlier in the day, the police arrested the second female suspect carrying an Indonesian passport. The arrest of a female national was later confirmed by the foreign ministry of Indonesia.
She was identified with the help of CCTV footage from Kuala Lumpur International Airport, the police said.
Malaysian police made the first detention of a woman holding Vietnam travel papers on Wednesday.
Police said they were looking for a "few" other foreign suspects as part of the ongoing investigation into the death of Kim Jong-nam.
TRT World's Melanie Ralph in Singapore has more on these developments.
Target of assassination?
Malaysian media reported that the post-mortem on Kim Jong-nam's remains was completed on Wednesday evening.
The 46-year-old died under mysterious circumstances as he prepared to catch a flight to Macau, a Chinese territory.
Kim was known for speaking out against his family's hold on North Korea.
South Korean intelligence services said Kim Jong-un had issued a "standing order" for his half-brother's assassination.
There had been a failed attempt in 2012 to kill Kim, South Korean lawmakers said.
Malaysia is in discussions with North Korea about the release of Kim's body.
"After all the police and medical procedures are completed we may release the body to the next of kin through the embassy," Malaysia's Deputy Prime Minister Ahmad Zahid Hamidi said on Thursday.