Fellow K League side FC Seoul endured a miserable night in the Korean capital, however, going down 3-1 to Japan's Gamba Osaka.
Seongnam are the only one of South Korea's four teams in Asia's elite club competition to come away with a home win after Jeonbuk drew 1-1 with Beijing Guoan and Suwon went down 3-2 to Kashiwa Reysol on Tuesday.
Seongnam went ahead in the 23rd minute at Tancheon Sports Complex after Kim drifted into the box and fed Jorginho, who fired home first time.
There seemed little danger when Guangzhou's Huang Bowen picked the ball up 30 yards from goal but the midfielder cracked home a stunning strike to bring his side level just before the break.
Guangzhou were reduced to 10 men in the 65th minute when Li Xuepeng was dealt as straight red for raking his studs down the arm of Ricardo as the two contested a high ball.
Just when it seemed the Chinese side would hold on for the draw, Seongnam were awarded a penalty when Ricardo went down in the box, though it was difficult to see where the contact occurred.
While several of his team mates could not bear to watch, captain Kim shrugged off the pressure to fire home the spot kick and seal the win.
After a cagey first half in Seoul the home side looked to push the pace in the second and get captain Cha Du-ri more involved in the game, but with the marauding right back pushing forward Gamba found space to exploit.
Neat interplay down the left saw Hiroki Fujiharu reach the byline and cut the ball back for Takashi Usami, who crashed in a volley to give Gamba the lead in the 62nd minute.
The Japanese side went 2-0 up 11 minutes later after Fujiharu again found space down the left and his deep cross was met first time by Koki Yonekura.
Usami hammered the nail into Seoul's coffin four minutes from time by holding off two defenders to score Gamba's third, before Yun Ju-tae grabbed a consolation goal in injury time for the dispirtied Koreans.
The return legs are played next Wednesday. (Reporting by Peter Rutherford; Editing by Pritha Sarkar)