The Honda 350cc was being ridden by two men on Ninian Park Road at 21:30 BST on Friday when the crash happened.
One man was found with serious injuries and was taken to hospital. The second man left the scene but was located a short time later and is helping police with their inquiries.
The road was closed for four hours while investigations took place.
A man was seriously injured after a motorbike mounted a kerb and hit a wall in Cardiff.