Boro have been in bad need of numbers in defence after allowing Bernardo Espinosa and Alex Baptiste leave the club this summer.
Garry Monk's side have fielded 19-year-old academy graduate Deal Fry in all but one of their games this season and Shotton is expected to come in as an experienced alternative.
Midddlesbrough are thought to have spent around £3m on the former Stoke City man whose move turned into a rather protracted affair as Birmingham struggled to find an alternative.
#Boro are delighted to announce the signing of @RyanShotton88 from @BCFC!
— Middlesbrough FC (@Boro) 30 August 2017
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