Internacional collected the third-place consolation prize to end their disappointing Club World Cup campaign in Abu Dhabi.
The Brazilian side had been expected to meet fellow seeds Inter Milan, the European champions, in the final.
However, they suffered a shock defeat by TP Mazembe in the semi-final and had to console themselves with today's appetiser before Inter and Mazembe contested the main event.
Internacional were already 2-0 up through Tinga and Alecsandro's goals before Jang Suk-Won was sent off for Seongnam, making the Korean side's task even harder.
The Porto Alegre side made hay after the break with Andres D'Alessandro making it 3-0 before Alecsandro clinched his second of the game.
Seongnam then stunned Inter with Colombian striker Mauricio Molina's brace in the last six minutes, though it proved immaterial as the Latin Americans ran out comfortable winners.