She was helping him cross an overhead bridge, but he abused the teenager's kindness and molested her.
Yong Kiam Khim, 59, who is jobless and has difficulty walking, was jailed for three months yesterday after pleading guilty to outraging the 16-year-old girl's modesty.
A second molestation charge, involving a 20-year-old woman, was taken into consideration during sentencing.
According to court papers, Yong outraged the woman's modesty at around 11.40pm on Sept 2 last year, by forcefully putting his arm over her shoulder and holding her hands on board SBS bus service 105 while travelling along Toa Payoh Central.
He was out on bail for this case when he struck again on April 10.
Deputy Public Prosecutor (DPP) Eunice Lau said the teenager, who cannot be named to protect her identity, was walking down the overhead bridge along Dunearn Road at around 4.45pm that day when Yong approached her.
He told her: "Help me." She agreed to help him cross the bridge.
She offered him her arm and he held on to her. He also placed his hands on her shoulder and waist for support.
DPP Lau said that when they reached the other side, he suddenly hugged the teenager, kissed her cheek and thanked her for helping him.
She thought that he was simply expressing his gratitude and helped him down the stairs.
But Yong kissed her cheek again and this time, she asked him to stop. However, thinking that he would stop trying to kiss her if she allowed him to do so one more time, she relented.
After kissing her left cheek, he tried to kiss her mouth by turning her face towards his. She pushed him away.
Despite this, the girl still continued to help Yong walk down the stairs.
Further taking advantage of her kindness, Yong then touched her right breast and pressed his whole body against hers after holding her waist tightly with his hands.