He was airlifted to Yorkhill Children's Hospital after being hit by a car.
Police said the little boy was walking with his mother in Barnes Terrace, near to the junction with Barns Park, in Ayr yesterday afternoon.
As he was crossed the road, he was hit by a car.
Emergency services raced to the scene, shortly after 3pm yesterday, and an air ambulance was scrambled to get the four-year-old to hospital.
Doctors today described his condition as "critical". His family are understood to be at his bedside.
Police said the woman driver of the car was not injured but has been left "very upset" by the incident.
Accident experts closed the road for several hours as investigations got underway.
Sergeant Iain Pittams, from the Divisional Roads Policing Unit, is leading the investigation into the crash.
He appealed for anyone who was in the area of Barns Park, at around 3pm yesterday, to contact him.
A Police Scotland spokeswoman said: "At around 3.05pm on Monday, a four-year-old boy was walking with his mother in Barnes Terrace, Ayr when he tried to cross the road.
"As he was crossing the road, he was struck by a car.
"Emergency services were contacted and the boy was airlifted to Yorkhill Children's Hospital.
"He is detained in what is described as a critical condition."
Anyone with any information is asked to contact Police Scotland on 101.