The victim later told her father the star was ‘a dirty old man’, the court heard.
Harris was taking part in a TV show which was filmed on Jesus Green, Cambridge, in the summer if 1978.
The jury at Southwark Crown Court was told that he had already been convicted of groping another 16-year-old at a previous hearing.
The show involved a number of celebrities taking part in a number of sports, and others present included Michael Aspel, Tony Blackburn and Rula Lenska.
The alleged victim, who was 16 at the time, was a helper at the event and was chosen as a “gofer” - assigned to help a particular celebrity, who on this day was Harris.
Jonathan Rees QC said: “At one point during the day, he was praising her for working so hard.
“He asked to ‘come here’ and put one arm around her from the front while his other hand deliberately cupped her breast.
“The victim thinks Mr Harris may have told her that she was ‘a bit irresistible’.”
Later in the day a car was arranged for Harris to take him to a swimming pool for one of the shows events and asked the victim to get in for a lift.
The prosecutor said: “She sat beside him on his left. Once she was inside, Mr Harris told her she was lovely and that she had worked so hard.
“According to the victim he slid over towards her on the back seat and put his left hand on her knee, before sliding his hand up her thigh over her jeans.
“She picked his hand up and pushed it away. It was at this point that the driver entered the car.”
The jury were told it was the touching in the back of the car which forms the count of indecent assault.
When she got home she told her father what had happened and said: “Rolf Harris was a dirty old man.”
Since the alleged incident the court heard the victim had told a number of close friends and relatives she had been indecently assaulted.
Mr Rees said: “It is understood that in respect of this allegation, there is an issue as to whether Mr Harris would have been in the ‘direct company’ of the victim for the length of time she claims.
“In addition he denies that he acted in the way alleged and contends that he would not have acted in such a way.
“The prosecution say one aspect of the evidence that supports the allegation being made by the victim is that one of the indecent assaults that Mr Harris was convicted of at the conclusion of the earlier trial involved him groping the bottom of a 16 year old girl during the very same event, namely the production of Star Games on Jesus Green, Cambridge, in the summer of 1978.”
Harris, formally of Berkshire, denies the seven counts of indecent assault and one alternative count of sexual assault.
The trial continues.