The 26 year-old will link up with Boro after failing to break his way into Steve Bruce's plans this season.
The defender will go straight into Boro's squad to face Sheffield United at the Riverside tomorrow.
The club have also confirmed that midfielder Josh Walker has joined Watford on a free transfer.
The 21-year-old, who goes straight into Watford's squad for their Championship game at Hull City today, has signed a two-year contract at Vicarage Road with an option for a further year.
Boro will receive a percentage of any fee should Watford sell Walker in the future.