The move came hours after dozens of South Korean firms threatened to withdraw from the zone at Kaesong in the North, complaining they had fallen victim to political bickering between the two rivals.
"The hotline was restored this afternoon after North Korea accepted our request to normalise it," a South Korean unification ministry official said on condition of anonymity.
The move followed months of high tensions and threats by Pyongyang of nuclear war.
As tensions began easing last month, the North restored the hotline across the border truce village of Panmunjom for talks on setting up a high-level meeting to discuss the fate of the zone.
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The Kaesong estate, where North Koreans work in Seoul-owned factories, was the most high-profile casualty of the months of elevated tensions that followed the North's nuclear test in February.
Operations at the complex just north of the border ground to a halt soon after the North banned entry by the South's factory managers and other officials on April 3.
About a week later Pyongyang pulled all its own workers out.
In an unexpected change of course today, the North sent a message to the South saying South Korean businessmen and managers would be allowed to visit the complex.
South Korean managers could have talks with their North Korean counterparts during their trip, it said.
The North's invitation sparked speculation that it may be ready for dialogue with the South.
But South Korea responded cautiously, saying it would review the proposal and convey its response later.
"The North's offer is seen as a strategic change in its attitude, but we cannot say yet it is interested in sincere talks with the South," Yang Moo-Jin of the University of North Korean Studies in Seoul told AFP.
Kaesong was established in 2004 as a rare symbol of cross-border cooperation, with some 53,000 North Koreans working for 123 South Korean firms in recent months.