Officers received reports of a single-vehicle collision on Gainsborough Road, Bishopbridge just after 7pm involving a white Yamaha FZR 600 motorcycle.
A 34-year-old man sustained serious injuries and was taken to hospital where he remains.
The A631 junction, just north of West Rasen, was closed before 8pm to allow emergency services to deal with the incident.
It was reopened during the early hours of this morning (Thursday).
If anyone witnessed the collision or captured dashcam footage, please get in touch with officers.
You can contact Lincolnshire Police by calling 101, quoting incident 421 of May 18, or by emailing [email protected]
If you wish to remain anonymous, you can report information via the independent charity CrimeStoppers on 0800 555 111 or online via the website at https://crimestoppers-uk.org/.