The 27-year-old - a two-time PGA Tour winner who has starred in the Presidents Cup - is one of the main attractions at Woo Jeong Hills Country Club this week.
Following a 69 during his first round, Im, who started at the 10th, surged into contention thanks to a a flawless four-under front nine.
A bogey at the fourth blotted his card, but birdies at the fifth and seventh saw him take the outright lead on seven under.
However, he dropped a shot at the ninth to drop back to six under after a 67 and sit alongside fellow South Korean Song Min-hyuk (67), Frenchman Tom Vaillant (69) and England's Alex Fitzpatrick (67) who were already in the clubhouse on the same mark.
"I feel that today I was very consistent," Im said.
"Looking back at my game, I think I was aggressive when I needed to be and I played safe when I needed to ... My strategy was really good today.
"When we come to Korea we have all these fans who come out and cheer and support us. That really gives you the boost you need. When I come here, I really have a good time."
Spaniard Nacho Elvira carded a second successive 68 to make it a five-way tie at the top before New Zealand's Daniel Hillier (69) also joined the pack.
Lee Jung-hwan produced the round of the day with a brilliant 65 to increase the leading group to seven before overnight joint leader Li Haotong's (71) mixed round of four birdies and four bogeys meant he remained at six under.
David Micheluzzi (73) is the best-placed Australian at two under, while Adam Scott (67) is one under and Jason Scrivener (71) one over.
With AAP.
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