Bent, 31, has struggled with hamstring injuries and has not featured for Birmingham so far this season.
He has signed up for an initial two-month period with the Championship high-flyers.
Middlesbrough bound: Marcus Bent in action for Birmingham against Preston last season
Boro boss Gordon Strachan said: 'I'm delighted to bring Marcus in. He brings us ability, strength, pace and experience.'
Birmingham manager Alex McLeish added: 'Going up there for two months is an opportunity for him.
'He's obviously had one or two setbacks with injuries. But he's a big, strong centre-forward with pace and presence - and when he's on top form he's an asset.
'When it's time for him to come back, we hope he'll be in great form and pushing for a place in the team.'
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