The girl was attacked on the top deck of the 57 as it left Silverburn shopping centre in Pollok, Glasgow, at about 10:30pm on Friday 22 March.
She got on the bus with a friend and they took a seat downstairs. The victim went to the upper deck where she got into a conversation with two men who boarded the bus in the city centre.
She was then assaulted and raped by both of them.
Her friend, also 14, went upstairs to see her and raised the alarm with passengers downstairs. A woman and two men came to the girls’ aid. They all left the bus two stops along from Silverburn.
The passengers are said to have waited with them until they got another bus home, thought to be in the Darnley area.
No CCTV footage is available from the bus, while Strathclyde Police are appealing for those passengers to come forward to help their investigation.
First Glasgow, the bus operator, said the attack was not reported to the driver and the company is working with police to find the men involved.
A spokesman said: “Unfortunately, the incident took place on the top deck of the bus and therefore out of our driver’s eyeline and no-one on board approached our driver to report a disturbance.
“The driver ejected the two males some time later in the journey – this was due to separate unruly behaviour. Our thoughts are very much with the victim of the assault.”