The accident happened at 10.20am in London Road, close to the junction of Birkenhead Avenue and Wickes DIY store.
Police and paramedics were called, and the London Air Ambulance landed in Tiffin Boys' School to assist.
A Kingston police spokeswoman said: "Police confirm that they were called at 10.20am to a road traffic accident on London Road at the junction with Birkenhead Avenue.
"Officers are currently on the scene. London Ambulance Service is also in attendance.
"The Air Ambulance is en route to attend a man who is thought to have life threatening injuries."
An ambulance spokeswoman said the man, believed to be in his 50s, was being taken to St George's Hospital in Tooting.
London Road is closed in both directions and buses are being diverted.
Becky Mills, who works nearby in St Peter's Church as a fundraiser for the Joel Community Trust, said: "It's all been cordoned off and all the police vehicles are parked near Wickes.
"There are a number of emergency vehicles and a massive amount of traffic."