The 14-year-old sat downstairs with a friend but then moved to the upper deck where she was assaulted and raped by two men.
Police have appealed for other passengers to come forward.
Detectives said the girl engaged in conversation with two men who had boarded the bus in the city centre.
Her friend, also 14, went up 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 the girls until they got another bus home, thought to be in the Darnley area of the city, where police were contacted.
'Frightening experience' No CCTV footage is available from the bus.
Both of the attackers are said to be aged around 18 to 20.
One is about 5ft 10in, with a shaved head and wearing a black jumper.
Det Con Jackie Carroll Strathclyde PoliceShe has been left traumatised by her ordeal”
"She has been left traumatised by her ordeal.
"We're continuing extensive enquiries into the incident, and would like to trace two men and woman who came to her assistance, as they will have vital information which could help with our investigation."
Police said the two men were very loud and using crude and lewd comments to other passengers.
The bus driver ejected them at Pollokshaws Road, near to the park, at about around 00:30 hours.
A spokesman for bus operator First Glasgow said: "Our thoughts are very much with the victim of the assault."
"We are assisting the police with their investigation and would echo their appeal for anyone to come forward who may have information about the incident.
"The safety and security of our staff and passengers is our number one priority and we will continue to do everything we can to help the police apprehend the perpetrators."