About 1,000 pairs were attached to the five large gas-filled balloons, which were launched in the northern South Korean city of Paju.
The Seoul-based group North Korea Peace said the messages sent with each pair of socks were "politically innocuous."
"We're not interested in sending political messages or sparking any troubles there. All we want is that people in the North wear warm socks over their frozen feet," Sunny Kim, a spokeswoman for the activists, told .
"Warm socks are so rare and they can easily be traded for cash in the North. One pair of socks fetches about 22 pounds of corn, which is enough to sustain a person for a month," .