Family members will get to spend a few hours each day together over three days.
Most of those taking part are elderly people who are eager to see their loved ones once more before they die.
A South Korean participant for a reunion sits inside a bus as she arrives at the South's CIQ (Customs, Immigration and Quarantine), just south of the DMZ in Goseong, South Korea, August 20, 2018.
The 89 ageing South Koreans, dressed in their best suits in the scorching sun, hobbled one by one to 14 coaches in the South Korean port of Sokcho - wheelchairs alongside the vehicles - some excited and others in a state of disbelief.
The reunions were included in the Panmunjom Declaration signed by South Korean President Moon Jae-in and North Korean leader Kim Jong-un on April 27, in which the countries agreed to "endeavor to swiftly resolve the humanitarian issues that resulted from the division of the nation" and "proceed with reunion programs for the separated families" on the occasion of the August 15 Liberation Day, when a then-unified Korea gained independence from Japan's colonial rule in 1945.
Min Byeong-hyun, 82, couldn't believe his eyes when he saw his younger sister, Dok-yo, 72.
Most of the reconnected families had not seen each other for at least 65 years.
"I tried so hard, too, searching for you for seven years", Ri told his brother.
"I stayed alive to meet you, oppa [older brother]", Yu-dok said, as she whipped out a school graduation photo of him.
"(My family) in North Korea didn't live long so I prayed for my son's health", she said.
But as those who have been separated grow old, time is running out.
The two Koreas, which are technically still at war, have organised reunion events before, but this is the first in three years.
In a statement Monday, President Moon urged both Koreas to work towards more reunions in future, noting his own family history as the child of North Korean refugees.
A total of 100 people were chosen on each side to attend the reunion, but some dropped out after realising the relatives they had hoped to see were no longer alive.
After 11 hours together over the next three days, the pair will part, nearly certainly never to see each other again, and - unless something changes - they won't even be able to exchange letters. Many participants are elderly, with 35 members of the South Korean group older than 90 and the oldest member at 101 years old.
Another 337 South Koreans will be granted reunions later this week.
Many brought gifts of clothing, medicine and food for their North Korean relatives, since anything deemed extravagant by Pyongyang was unlikely to pass muster.
The two leaders met a second time in May, where they agreed to resume family reunions, and are expected to meet again in the coming weeks.
But while Kim and US President Donald Trump held a landmark summit in Singapore in June, Pyongyang has yet to make clear what concessions it is willing to make on its nuclear arsenal, while Washington is looking to maintain sanctions pressure on it.