Rolf Harris put his hand up the skirt of a 12-year-old girl as she stood inches away from her mother, before giving the child an autograph, a court has heard.
The woman, the youngest of the alleged victims in a second raft of indecent assault charges the entertainer faces, told his trial he touched her "where no-one had touched me before, not even myself".
Giving evidence about the incident which she said happened in Portsmouth in 1977 when she went with her mother to get the Australian celebrity's autograph, the woman, who described herself as "completely unworldly" at that time, said: "It just felt horrible, it just felt wrong."
Harris is alleged to have said "let me give you a little cuddle" before assaulting the young girl as her mother stood less than a foot away.
The woman, who appeared via video-link at London's Southwark Crown Court, said she had initially been reluctant to tell anyone else about what happened after claiming her mother dismissed her account of the incident, which she said lasted seconds before she pulled away from the star.
She told the jury: "She (my mother) said something like 'Don't be so stupid'".
She added: "I felt like if my mum didn't believe me then who will believe me?"
Harris is accused of assaulting seven girls and women in a series of "brazen" attacks spanning 30 years, the most recent in 2004.
Harris, who is serving a sentence at Stafford Prison for a series of offences of indecent assault carried out on four female victims, is appearing by video-link.
The 86-year-old has pleaded not guilty to the seven fresh counts of indecent assault and one alternative charge of sexual assault.