The 30-year-old played 21 times as the Tigers won promotion to the Premier League but has found games hard to come by this term, making just one league appearance.
McLean can expect plenty of game time at Birmingham, who are short of options up front after Matt Green was ruled out for the rest of the season with a knee injury.
He goes straight into the squad for Saturday's visit of Blackpool.
McLean confirmed the move on his Twitter account, saying: "Just signed on loan at Birmingham City till Jan.. Looking forward to it. Can't wait to get started #BCFC."
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- Premier League