The jury in his rape and indecent assault trial at Preston Crown Court was directed by the judge to return a not guilty verdict on the single count of indecent assault.
The allegation was one of two made by a woman who said she was aged 14 when Roache, who plays Ken Barlow in the ITV soap, twice made her perform a sexual act on him.
Giving evidence, the complainant said the actor first struck in the gents' toilets at Granada Studios in Manchester in the summer of 1965.
Jurors were then shown a letter and signed photograph that Roache, 81, had sent to her after that alleged incident in which he asked to her write back to him when she returned to school.
The woman claimed later that same year she was picked up by the actor from the studios in his Rolls-Royce and they had a conversation of a sexual nature.
She said she thought that she had also been indecently assaulted in his car but she had ''no actual memory'' of the episode.
The judge in the case, Mr Justice Holroyde, said: "In relation to that episode the witness was not giving evidence that it did happen, she was giving evidence that she was thinking it did happen and that is not a sufficient evidential basis for the conviction of an offence."
Roache, 81, of Wilmslow, Cheshire, remains on trial over two counts of rape and four counts of indecent assault involving the five complainants who were aged 16 and under on dates between 1965 and 1971.
He denies all the charges.
After closing the prosecution case, Anne Whyte QC told the jury: "We do not invite you to convict on any allegation where the witness has no recollection. So we do not proceed against Mr Roache in relation to count 2."
The allegation on count 2 was that Roache between June and September 1965 assaulted a female person aged 14.
Mr Justice Holroyde told the eight female and four male jurors: "Each charge is a separate allegation which requires your separate consideration and separate verdict.
"The prosecution acknowledge quite rightly that in law the evidence which (the alleged victim) gave cannot be sufficient for a jury to find a guilty verdict.
"The prosecution are no longer pursuing that charge. Mr Roache is entitled to a not guilty verdict on that charge."
He said it was "a matter of law" that the jury "could not and must not" convict on the count concerned.
A female juror performed the formalities and entered a not guilty verdict when asked by the clerk.
The trial continues at 10am tomorrow when the defence case will start and Roache is expected to give evidence from the witness box.