The 14-year-old was attacked on the top deck of the 57 bus as it left Silverburn shopping centre in Pollok, Glasgow, at about 10.30pm last Friday.
Initially the girls took a seat downstairs but the victim went to the upper deck where she got into a conversation with two men, who both assaulted and raped her.
Her friend, also 14, went up to see her and raised the alarm downstairs, prompting a woman and two men to come to their aid. Strathclyde Police are appealing for those passengers to come forward to help their investigation.
Anyone with information can call Greater Glasgow CID on 101.