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.
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Goalkeeper Cammy Bell has signed a two-year contract with Kilmarnock after Dundee United allowed him to leave as a free agent.