The attack left him with a gash of about 1 cm on his neck, YTN television reported. Hospital officials did not immediately confirm the details of his injury.
Lee was airlifted by a fire department helicopter to the Pusan National University Hospital, a party official and emergency officials told Reuters.
"He is conscious and receiving treatment at the hospital," a Democratic Party official told Reuters.
The assailant appeared to be a man in his 50s or 60s, who wore a paper crown with Lee's name on it, news photographs showed.
The man approached and asked for an autograph as Lee spoke among a throng of supporters and reporters, then lunged forward and attacked him, video footage showed.
President Yoon Suk Yeol condemned the attack and instructed best care be given, his office said.
"This type of violence must never be tolerated under any circumstances," his office quoted Yoon as saying.
