The child is understood to be awaiting surgery in a Glasgow hospital following the incident.
The bull terrier, believed to have belonged to the boy's family, has been put down.
Police said the incident happened in Renton, West Dunbartonshire, on Tuesday evening.
Emergency services raced to New Cordale Road and the little boy was rushed to York-hill Hospital.
It is not clear if the incident happened in the street or in a property.
A Police Scotland spokeswoman said: "We received a report of a five-year-old-boy having sustained an injury after being bitten by a dog in Renton.
"The child was taken to York-hill Hospital where he has been detained for treatment."
The terrifying incident comes a week after a toddler was savaged by a pit bull.
The three-year-old is understood to have been playing in the garden of a house when the dog attacked him, causing facial injuries.
Police said incident happened at around 8pm last Tuesday in the Bellfield area of Kilmarnock.
Paramedics rushed to the semi-detached house in Campsie Road and took the youngster to Crosshouse hospital.
He had surgery for facial injuries.
A 25-year-old man is the subject of a report to the Procurator Fiscal in relation to the Dangerous Dogs Act.
The Government is considering measures to improve the safety around dogs which are known to be dangerous after a spate of attacks in recent years.