The 17-year-old victim was riding in Normanton Spring last night when his bike was hit from behind by a car, which is believed to be a Kia, and he was sent flying.
Police said three males then jumped from the vehicle and attempted to punch him before grabbing his bike and driving off.
Officers were called at 12.35am today, Saturday, July 17, to the junction of Normanton Spring Court and Normanton Hill, where the robbery reportedly happened.
A spokeswoman for South Yorkshire Police said the teenager, whose bike is estimated to be worth around £3,000, was not believed to have required hospital treatment.
She added that there is no description of the three attackers at this time.
The victim of that crime, a 31-year-old man, was hit by a car on Birley Spa Lane on Wednesday, July 14, at around 3.35pm, and left with minor injuries.
Police said that at this stage the two crimes are not understood to be connected.
Anyone with information is asked to call police on 101, quoting incident number 31 of July 17 in relation to the latest offence or incident number 506 of July 14 if calling about the earlier robbery.
You can also contact Crimestoppers, free and anonymously, on 0800 555 111.