Mr Harris, 87, is accused of touching the schoolgirl after he had filmed an episode of the BBC's Saturday Superstore in 1983, jurors were told.
Prosecutors alleged the Australian-born star then said: "Do you often get molested on a Saturday morning?"
Mr Harris denies four charges of indecent assault against three girls between 1971 and 1983.
Prosecutor Jonathan Rees QC told the jury Mr Harris had approached the girl in a room after filming of the show - which also featured pop group Wham! - had ended.
Mr Rees said: "She recalls him sliding his open right hand across her back over her clothes and touching her breast.
"He then said 'Do you often get molested on a Saturday morning?', which the prosecution suggests clearly shows the sexual nature of the touching."
Mr Harris, who appeared via videolink at Southwark Crown Court, is also alleged to have indecently touched a 14-year-old girl in 1971 and twice groped a third teenage girl in 1978.
The trial is due to last three weeks.
Former TV star Rolf Harris touched a 13-year-old girl's breast after filming a children's TV show, a jury has heard.