South Yorkshire Police said the 16-year-old was walking down a grass verge near The Lane, in Herringthorpe, at around 10pm when she was approached by an unknown man.
The force said its investigation is now a priority and patrols in the area have been increased following the sex attack.
Temporary Detective Inspector Graham Stead said: “Last night it was reported the 16-year-old had been walking along Badsley Moor Lane between 9.30 and 10.30pm, before walking down a grass verge next to a service road on The Lanes, in the Herringthorpe area of Rotherham when she was approached by an unknown man.
“An investigation has now been launched and specialist dedicated officers are currently working to identify the exact circumstances of the incident.
“The investigation is a priority for police and a number of lines of enquiry are now being pursued to gather CCTV from the area, conduct house to house enquiries and speak to any witnesses who may have been in the area at the time.
“The 16-year-old is now receiving specialist support from officers and we are urging anyone who has any information about the incident to contact us.
“Extra patrols have now been put in place in the area and anyone with any information or concerns, is asked to contact us on 101 quoting incident number 1280 of 31 March. Information can also be passed anonymously to Crimestoppers on 0800 555 111.”