The man was injured in Cherry Orchard Road close to East Croydon station at just after 10am on Thursday.
A local business owner said they saw an air ambulance land in a nearby school to tend to the victim.
Police said the injured man was taken by ambulance to a hospital in south London where he remains in critical condition.
A police statement makes no reference to anybody else being involved in the incident.
A Met Police spokesperson said: “Police were called by the London Ambulance Service shortly after 10am on Thursday, 5 May to reports of an injured skateboarder on Cherry Orchard Road in Croydon.
“Officers attended. The man was taken by LAS to a south London hospital where he remains in a critical condition.
“The road has since reopened to traffic.”