The 23-year-old joined Livingston on a deal until the end of 2015-16 after leaving Dundee United in January.
He previously spent the first part of last season on loan at Almondvale Stadium, and made a total of 30 appearances for Livi last term.
McCallum, who has also played for Arbroath and Berwick, is the League Two side's 14th signing of the summer.
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Plymouth Argyle have signed former Livingston goalkeeper Marc McCallum on an undisclosed-length contract.