Manager Chris Funnell says he'll be an "unbelievable asset" for the Poachers.
"I’m beyond thrilled that Kallum has joined us as a player-coach from Lincoln United," said Funnell.
"He amassed over 550 appearance for that club and is the definition of club legend.
"Kal’s joined us in a hybrid role. He's got loads to offer on the pitch and he'll be an unbelievable asset off it, too. His drive, winning mentality and desire will prove huge commodities for us in the second half of the season. In addition to that, his person-centred approach and humour off it will only bond our changing room closer.
"We’ve signed a real local non-league legend and I’m sure he’s going to be a huge success on and off the pitch."
Smith played his final game for Lincoln United against Skegness Town on Boxing Day and was given a guard of honour by both teams.
His arrival at the Mortgages For Stadium is a huge boost for Boston Town as they begin 2025 just 4 points and 2 places outside the play-off zone.
Funnell has also signed young midfielder Kris Zaluzinskis on loan from Boston United.
Boston Town's next game is away at Deeping Rangers on Saturday 4 January, kick-off 3pm.