The 31-year-old Blues striker has moved to The Riverside on an emergency two-month loan deal.
Bent has not made one first team appearance this season and in total has made just 18 starts and scored three goals since his move to St Andrews from Charlton Athletic in the summer of 2008.
"This is a good step for me. Middlesbrough are a big, established club and everything about the club is geared to going back up," he told the official Middlesbrough website.
"The gaffer was a big influence on my decision too. He is very experienced and motivated, so I'm looking forward to playing for him."
Bent could make his debut for Boro, who will be under the leadership of new boss Gordon Strachan for the first time, in their home game against Plymouth Argyle on Saturday.Debut Apps (+ sub) Goals Yellows Reds 09 Aug 2008 18 (17) 3 1 0 01 Sep 2007 26 (6) 7 1 0 22 Jan 2006 38 (14) 5 0 0 15 Aug 2004 44 (22) 8 3 0 15 Sep 2003 30 (5) 10 3 0 25 Nov 2001 56 (13) 23 5 0 25 Nov 2000 24 (17) 11 2 1 56 (0) 8 2 1 1 (1) 0 0 0 First App. Apps (+ sub) Goals Yellows Reds 09 Aug 2008 18 (17) 3 1 0 11 Aug 2007 30 (6) 8 1 0 19 Aug 2006 22 (13) 2 0 0 09 Aug 2005 22 (15) 3 1 0 15 Aug 2004 34 (8) 7 2 0 09 Aug 2003 34 (6) 11 3 0 10 Aug 2002 28 (9) 12 3 0 18 Aug 2001 25 (12) 10 4 1 12 Aug 2000 44 (8) 19 2 1 02 Oct 1999 36 (1) 0 0 0