Lee Min-bok said he had carried out the launches at night four times since January, most recently on April 4.
The Seth Rogen comedy, about a fictional CIA plot to kill North Korean leader Kim Jong-un, enraged Pyongyang.
Sony initially pulled The Interview after a hacking attack and threats to attack cinemas which were screening it.
But it changed its mind and gave the film a limited cinema release after being accused of responding to an attack on free speech.
The FBI says North Korea was behind the hack and threats, though it denies this.
Lee Min-bok, a defector from North Korea, said he had tied the DVDs to balloons along with bundles of US dollars and leaflets criticizing Kim Jong-un’s regime.
He told AFP news agency: “I launched thousands of copies and about a million leaflets on Saturday, near the western part of the border.”
He said the launches were all done in remote areas and without publicity but that the police “would have no right to stop me”.
Lee Min-bok told CNN, which joined him on Saturday’s launch, that he had not laughed at The Interview and found it vulgar.
But he said North Korea “hates this film because it shows Kim Jong-un as a man, not a God” and that he wanted to “tell the truth” to North Koreans.
Any North Korean who had access to a DVD player and was found to have watched the film would likely face a lengthy sentence in a prison camp.
South Korean activists have repeatedly carried out balloon drops across the border of material which they say shows the reality of life outside the restrictive country, in the hope of encouraging North Koreans to reject propaganda and stand up to their leadership.
North Korea has demanded South Korea stop such launches, saying they are provocative. Its border guards have in the past tried to shoot down the balloons.
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