SEOUL, South Korea — He clapped and he smiled, even posing for a group photo with a K-pop band.

The appearance by North Korea’s leader, Kim Jong-un, on Sunday at a concert by South Korean musicians in Pyongyang was all the more unusual because his authoritarian government has been struggling to stave off what it sees as an infiltration of the South’s pop culture among his isolated people.

But Mr. Kim shook the hands of members of South Korea’s most popular girl band, Red Velvet, which he and his wife, Ri Sol-ju, watched from a balcony.

After watching Red Velvet perform, Mr. Kim reportedly pronounced the event a “gift for Pyongyang citizens.”

The two-hour show at the East Pyongyang Grand Theater was part of a political détente between the neighboring countries as relations have thawed.