The highly symbolic outcome of recent efforts to improve inter-Korean relations saw dozens of cars, trucks and factory managers cross the border shortly after 8:30 am local time (0500 IST).
"I wish we could work together well again, just like before," said the 50-year-old manager of a Seoul textile company who declined to be named.
The optimistic mood at the Kaesong border checkpoint contrasted sharply with the sense of impending disaster that had loomed over the closure of Kaesong back in April.
As military tensions eased, the two Koreas agreed last month to work together to resume operations.
As part of the deal, the North accepted the South's demand that Kaesong be opened to foreign investors -- a move seen by Seoul as a guarantee against the North shutting the complex down again in the future.
"Honestly, I still feel a bit nervous because you never know whether the North will change its mind in the future," the textile company manager told AFP.
Born out of the "sunshine" reconciliation policy initiated in the late 1990s by then-South Korean president Kim Dae-Jung, Kaesong was established in 2004 as a rare symbol of inter-Korean cooperation.
It provided an important hard currency source for the impoverished North through taxes, other revenues, and its cut of workers' wages.
It had appeared immune to previous downward spirals in North-South relations, but finally fell victim to the months of tensions that followed the North's nuclear test in February.
Pyongyang initially barred South Korean entry to the park in early April and shortly afterwards withdrew its workers -- effectively closing down the complex, which houses production lines for 123 South Korean firms.
The most immediate task for the South Korean managers is to inspect the state of production lines that have been out of operation for nearly five months, and to determine how quickly they can get them up and running again.