The 14-year-old was assaulted near to Herringthorpe Playing Fields, Middle Lane, Clifton, on Wednesday, October 18.
South Yorkshire Police said there were two attackers involved.
A spokeswoman said: "Police are investigating the reported assault of a 14-year-old boy in Rotherham last week.
"Officers received a third party report of assault from a local hospital on Monday, October 23, after the teenager attended for treatment for serious facial injuries.
"It is understood that the teenager was walking home from school on Wednesday, October 18 when two male suspects reportedly assaulted him in Middle Lane, Clifton, near to Herringthorpe Playing Fields.
"Enquiries are ongoing to understand the full circumstances surrounding the incident, but anyone who may have witnessed the assault is asked to call."
Contact South Yorkshire Police on 101.