The 28-year-old man, identified only by his family name Lee, had been playing on-line battle simulation games at the cybercafe in the southeastern city of Taegu, police said.
Lee had planted himself in front of a computer monitor to play on-line games on Aug. 3. He only left the spot over the next three days to go to the toilet and take brief naps on a makeshift bed, they said.
"We presume the cause of death was heart failure stemming from exhaustion," a Taegu provincial police official said by telephone.
Lee had recently quit his job to spend more time playing games, the daily JoongAng Ilbo reported after interviewing former work colleagues and staff at the Internet cafe.
After he failed to return home, Lee's mother asked his former colleagues to find him. When they reached the cafe, Lee said he would finish the game and then go home, the paper reported.
He died a few minutes later, it said.
South Korea, one of the most wired countries in the world, has a large and highly developed game industry.