The crash happened at about 3.20pm on Friday near Leintwardine in south Shropshire.
Emergency services went to the scene on the A4110 near to the A4113.
As well as the land ambulance, medical crews with the Midlands Air Ambulance were scrambled from RAF Cosford.
A spokesperson for West Midlands Ambulance said the male casualty was being treated at the scene before a decision was made whether to airlift him to hospital.
Police warned motorists to avoid the area if possible with the collision affecting traffic travelling between Leintwardine and Wigmore.
A police spokeswoman said: "A motorcyclist suffered serious injuries following a collision with the trailer on a tractor on the A4110. We had been called shortly after 15:40."