A serial molester was on Thursday sentenced to 11 weeks in jail by a district court for outraging the modesty of a 12-year-old girl.

Student Phua Sze Son, 22, boarded a public bus on September 21 last year and sat next to his victim, who was in a school T-shirt and shorts and seated by the window.

Palm facing downwards, he slid his left hand under his victim's thigh and repeatedly wriggled his fingers against her shorts.

The girl got uncomfortable and decided to leave but had to use her bag to force her way out of the seat after Phua, who was blocking her path to the aisle, would not give way.

Handing down the punishment, District Judge Shawn Ho said that Phua's 12-year-old victim had been traumatised by the incident and was in tears when she rushed home and alerted her mother to what had happened. He also noted the victim's age and that this was the second time Phua had molested a girl aged below 14.

But the judge considered that Phua had pleaded guilty and appeared genuinely remorseful for what he had done.

Calling for a jail term of more than nine weeks, Deputy Public Prosecutor Terence Szetoh said Phua had continued to wriggle his fingers even when his victim, a vulnerable individual, tried to move away. The offence took place on public transport, he noted.

Phua had been convicted of four molestation offences since 2006. He has served punishments including probation and a reformative training term.

He could have been jailed for up to five years, fined and/or caned for outraging the modesty of someone under age 14.