Neil Poundford was given a 16-month sentence over the incidents in Wakefield.
Poundford, 58, was also made the subject of a sexual harm prevention order banning him from sitting near unaccompanied females on public transport for the next ten years.
Leeds Crown Court heard Poundford targeted the 17-year-old as she was travelling on the bus between Wakefield and Dewsbury on May 5 this year.
Robert Stevenson, prosecuting, said Poundford sat next to her and began talking to her,
He then grabbed her face and said: “I think I recognise you.”
Poundford then started to talk about driving lessons and touched the victim on her arm.
Mr Stevenson said the victim felt frightened and got up to leave then Poundford slapped her behind as she walked away.
Police obtained CCTV footage from the bus and were able to identify Poundford.
He was arrested and released on bail but targeted a 13-year-old girl at a bus stop in Horbury, Wakefield, on July 27.
Poundford approached the girl on Westfield Lane and made a comment about her jeans before touching her knee.
Poundford then showed her a bank book and said he had £3,000 in account. He also touched her face before she walked away and told a friend what had happened.
He was arrested for a second time and admitted what he had done.
Poundford, of Waterhouse Grove, Lupset, Wakefield, pleaded guilty to two offences of sexual assault.
The court heard he has previous convictions for sexual assault.
Thomas Stanway, mitigating, said Poundford accepted that he must go to prison and would like help to address his offending behaviour while in custody.
Judge Geoffrey Marson, QC, said: “This was serious persistent behaviour.”
“Each offence must have caused your victim serious alarm. It must have been very troubling for them.
“You did it for your own sexual gratification.