The 30-year-old started his career at Killie, making 115 appearances before joining Rangers in 2013.
He moved to United in 2016 and made 35 appearances for the Tannadice side.
Kilmarnock manager Lee McCulloch told his club website: "He provides great competition for the other goalkeepers and I'm delighted to have him."
Dundee United had previously said Bell would not leave for nothing by posting on their Twitter feed that they were "open to offers".
They then relented, though, and "mutually agreed" the player's release.
Find all the latest football transfers on our dedicated page.

Malcolm Fyfield has suffered severe post traumatic stress disorder since surviving the flooded Gleision drift mine in September 2011.
He told Swansea Crown Court the only sign of water near the point being mined was "ponding" on the ground.
Mr Fyfield and owners MNS Mining Ltd deny manslaughter charges.
David Powell, 50, Charles Breslin, 62, Philip Hill, 44, and Garry Jenkins, 39, died when 650,000 gallons of water rushed into the area they were working in after they blasted into a flooded old section of the pit.
Mr Fyfield managed to escape and spent 12 days in hospital recovering from a broken hand and bruising and swelling as a result of a "near-drowning experience".
The former mine manager insisted he had not inspected the wrong part of the mine and said he was sure he was not mistaken because of the angle and gradient of the underground road.
The jury was told Mr Fyfield also suffers from "survivor guilt" and certain words used to describe the flooded mine trigger a feeling of being back in the pit.
Elwen Evans, QC, defending, told the court that "ponding" water found in the area before the controlled explosion was not unusual and Mr Fyfield did not think it was particularly significant.
Miss Evans reminded the jury Mr Fyfield was working alongside the men, adding: "It would have been incredibly reckless to make the breakthrough at a location he had not inspected, putting at risk the lives of four men as well as his own.
"We are dealing with someone whose reputation was as sky high as it comes.
"Year after year he did everything by the book. He knew how to do it, but this time he did not? It does not make sense."
Miss Evans asked the jury to decide the case according to the facts and to not base their verdicts on emotion.
The trial continues.
Goalkeeper Cammy Bell has signed a two-year contract with Kilmarnock after Dundee United allowed him to leave as a free agent.
[NEXT_CONCEPT]
The manager of a mine where four workers drowned checked three times for any signs of water or danger the day before it flooded, a court has heard.