The incident happened at Sutterton early yesterday morning at the junction of Blows Lane and Wigtoft Road.
The 63-year-old victim, who police describe as vulnerable, was out waling his dog between 5am and 5.30am.
Four men wearing high-visibility vests jumped out of a white van and approached him.
The men attempted to take the dog, and when the victim tried to stop them, they struck him with a weapon before fleeing.
The victim, who managed to stop them taking his dog, suffered broken ribs, minor cuts and a black eye.
Police are appealing for witnesses and anyone with dashcam footage.
Detective Sergeant Simon Dovaston said: “This is a despicable offence against a vulnerable man as he fulfilled one of his pleasures in life.
“While these offences are rare, I urge the public to come forward if they were in the area at the time or have dashcam footage that may assist the investigation.
“Our neighbourhood policing team are going out to do further reassurance patrols and I would also urge the public to remain vigilant to this type of crime that is unfortunately taking place in our neighbourhoods.”
Contact police by calling 101, quoting incident 104 of 20th July 2021, by emailing [email protected] using incident 104 of 20/07/2021 as the subject line or anonymously via the independent charity CrimeStoppers on 0800 555 111 or online.