The woman was sitting at the bus shelter in Cottingham, Hull, about 4.30pm on Thursday when the man attacked.
Police said the woman was left “shaken” after the alleged sexual assault.
A 21-year-old man has now been arrested on suspicion of sexual assault.
Humberside Police have now appealed for any witnesses to come forward, including a man and a woman who may have been nearby.
Detective Sergeant Tracy St Paul said: “I want to offer my assurance that whilst incidents of this nature remain incredibly rare in our area.
“We hope news of this arrest will assure the community that we are treating it as a priority.
“The woman has understandably been left shaken because of the incident and she is currently being supported by specialist trained officers.
“We are currently exploring all lines of enquiry available to establish the full circumstances and officers will be carrying out enquiries in the local area. If you see one of our officers and have any concerns, I encourage you to speak to them.”
If you think you may have been in the area around this time or have any information that may assist with our enquiries, please contact police quoting log 341 of 26 October.
Alternatively, you can call the independent charity Crimestoppers anonymously on 0800 555 111.