South Yorkshire Police said the 66-year-old woman was riding a black Monarch mobility scooter which was involved in a collision on Oldgate Lane at the junction with Doncaster Road just after 12pm on Wednesday January 13.
She was taken to hospital with serious injuries but her condition is now described as life-threatening.
No other vehicle is believed to have been involved.
A South Yorkshire Police spokesman said: “Officers want to hear from anyone who was in the Oldgate Lane or Doncaster Road area between midday and 12.20pm on Wednesday 13 January, who may have witnessed the collision.
“Please call 101 or email [email protected] quoting incident number 370 of 13 January 2016.”