The 23-year-old's arrival marks a breakthrough for the Teessiders, who have been largely frustrated in their attempts to add to their squad in recent weeks.
A versatile left-sided player, McQueen is expected to provide competition for Boro favourite George Friend at left-back and left wing-back, though he is also competent further forward, having started his career as a winger.
McQueen has 20 Premier League appearances to his name for 'Saints' and famously made his first start for the club in a Europa League victory over Inter Milan at the San Siro.
An England U21 international, McQueen is known for his combative nature and delivery from wide areas.
The defender is Boro's sixth summer signing following the arrivals of Aden Flint, Paddy McNair, Andy Lonergan, Jordan Hugill and Mo Besic.
McQueen will wear the number 14 shirt this season.
August 30, 2018