The former Port Vale forward was on trial at Glanford Park in pre-season, but did not do enough to be offered a long-term contract.
He rejected the offer of a monthly deal, and today agreed a six-month deal with the Imps.
The 33-year-old will make his debut against Rushden And Diamonds on Saturday.
Lincoln manager Keith Alexander said: "Adrian's an experienced player who has that little bit of nous that we need."
Littlejohn will make an early return to Glanford Park next weekend in the first Lincolnshire derby of the season.