Joseph Prunty, from Cardonald, Glasgow, raped a 12-year-old girl and sexually abused a teenager.
The 56-year-old attacked the child and the 15-year-old between 2006 and 2012 at various locations in Renfrewshire.
Detective Inspector Mark McGowan paid tribute to the young victims for reporting the "atrocious" crimes.
Bus driver Prunty raped the 12-year-old in a car in 2008 and at a house in 2012. He also groped the older girl.
Detective Inspector McGowan said: "These were atrocious crimes committed on young girls over a number of years.
"I cannot praise the victims highly enough for having the strength and courage to put their faith in my team and the judicial system.
"The memories of what they suffered at the hands of Joseph Prunty will never be erased."
At the High Court in Edinburgh, temporary judge Michael O'Grady QC told Prunty, who already had a sex conviction, that jail was the only sentence available to him.