The 21-year-old has signed a three-and-a-half year contract, subject to international clearance.
De Sart has made 66 appearances for Standard Liege since making his debut for the Belgian side in 2013.
CONFIRMED | Signing number three! Welcome to #Boro, Julien De Sart! https://t.co/5H8kKGpFhV pic.twitter.com/M1PkTLQFnm
— Middlesbrough FC (@Boro) February 1, 2016
