The road was closed for a total of six hours as officers carried out a full investigation of the scene.
They are now trying to speak to anyone who saw what happened.
A force spokesperson said: “At approximately 7.07pm on Tuesday a single-vehicle road traffic collision occurred on the B3417 Lee Moor.
“A motorcyclist who is a 49-year-old male from the Launceston area had been travelling in the Plympton direction and lost control on a right-hand bend.
“The male rider sustained serious injuries and was taken to Derriford Hospital where he remains in a critical condition.”
Any witnesses are asked to call 101 quoting log 803 of 17/09/19.