Sheffield Crown Court was told how it took Terence Chapman's victim until 2016 to pluck up the courage to report the incident to police, which took place in the early 00s when she was aged between 15 and 16-years-old.
"She thought for a long time that it was her fault, but eventually came to realise that he chose to assault her at a time when she was vulnerable," said Andrew Bailey, prosecuting.
In a victim impact statement read out in court, the woman, who cannot be named for legal reasons, said: "I was terrified, but over the years I have been able to put it out of my mind."
Mr Bailey told the court that Chapman, of Longley Hall Grove, Longley sexually assaulted his victim on a bus travelling out of Sheffield City Centre and then assaulted her again in an alleyway located close to the bus stop where she alighted from the vehicle.
Chapman was arrested in January last year, and initially said he could not remember much of what had happened around the time of the offence due to drinking to excess but did not think he would have committed the offences.
He pleaded guilty to two counts of indecent assault at a hearing held last month.
Defending, Joy Merriam, told the court: "When the defendant did plead guilty he did so because he didn't wish the complainant to go through the ordeal of giving evidence.
"He finally accepted at that point that there must have been something he had done, although he doesn't remember because of the amount of alcohol he was drinking."
Judge David Dixon sentenced Chapman to five months in prison, suspended for two years, made him the subject of notification requirements and ordered him to complete a sex offenders' treatment programme.
He said: "Regrettably, like for a lot of women assaulted in this way, she blamed herself. But she wasn't to blame - this was you, completely."