An investigation has been launched after an eight-year-old child was hit by a car in Sydney's west overnight.

It's believed that the boy was crossing Amy Street in Regents Park at about 8pm on Monday night when he was struck by a vehicle.

He was rushed to Westmead Children's Hospital with head injuries, where he remains in a serious but stable condition.

The driver was taken for mandatory blood testing and officers are looking into the circumstances surrounding the incident.