On May 16, 2018, news broke that Lee was charged with sexual harassment and threatening a female celebrity with a weapon. Lee was charged and investigated on April 8, before the police sent his case to the prosecution with a recommendation of indictment. According to authorities, Lee was drinking with said female celebrity when he attempted to kiss her and make physical contact, but was rejected. After continuous attempts for physical contact, the victim called her boyfriend to ask for help, which caused Lee to threaten her with a knife. Following the incident, Lee remained silent and continued filming the 2018 television series About Time and host Music Bank. Lee was supposed to appear in About Time, but was pulled out following news of the criminal charges. He was also removed from his role as the host of Music Bank.
On May 24, 2018, Lee apologized for his actions while visiting the Seoul District Public Prosecutors’ Office for interrogation. On July 12, the first day of trial at the Seoul Eastern District Court, Lee plead guilty to charges of sexual harassment and criminal threats. Lee's attorney stated that they acknowledge guilt since Lee's DNA was detected on the victim's ear and he was apprehended by police while holding a knife. However, he also stated that Lee was "unable to remember anything from the incident" and was "not in control of his body due to intoxication", and requested a lenient sentence due to Lee's "diminished mental and physical faculties" cause by intoxication.