The assault happened at around 8.45pm on Sunday when a man in his 20s was knocked off his bike on London Road.
His attacker, who is believed to have got out of a car, then punched and kicked him repeatedly while he was on the floor.
A spokesperson for Lincolnshire Police said: “It is believed that the offender was driving along London Road and that he left his vehicle, ran towards the victim, assaulted him, and returned to his car to leave the scene.”
The victim sustained minor injuries.
The offender is described as a white male who is of a large build, with dark facial hair, shaved hair, and a tattoo to his right arm.
The spokesperson said: “If you know who he is or if you witnessed the incident yourself, please get in touch.”
Contact the force by calling 101 quoting incident 462 of 29th August; emailing [email protected] with incident 462 of 29th August in the subject line, or report anonymously, you can do so through the independent charity, CrimeStoppers on 0800 555 111.