Nathaniel Mendez-Laing ended a five-month hunt for a new club by signing for Boro, after previously being released by Cardiff City back in September. The ex-England youth international, now 28, will be looking to reignite his career at the Riverside Stadium, and recapture the form that promoted Neil Warnock to sign him for the Bluebirds back in 2017.
After a host of additional signings were linked with a switch to Middlesbrough, Fulham winger Neeskens Kebano was snapped up on a half-season loan deal. He's been capped at senior level for the Democratic Republic of Congo, and has previously turned out for the likes of Genk and French giants Paris Saint-Germain.
Meanwhile, Boro are gearing up for their league clash against Brentford this weekend, which is being billed as real 'six-pointer' in the race for the play-offs, with both sides still well in contention for promotion.
When the two sides last met, back in January, the Bees knocked Warnock's side out of the FA Cup with a 2-1 win, and Boro will be eager to get revenge on Saturday afternoon.
Here's our round-up of the biggest transfer deals from yesterday's deadline day, including Middlesbrough and a number of their divisional rivals.
5. Hornets sign big-name duo
Watford raised eyebrows with the capture of the sons of both Denis Bergkamp and Mauricio Pochettino. The former, Mitchel, and the latter, Maurizio, will both be integrated into the Hornets' U23 side. (Club website) Photo: Richard Heathcote
6. Field heads to QPR
West Brom midfielder Sam Field joined QPR on a loan deal. The ex-England U20 international spent last season on loan with Charlton Athletic. (Club website) Photo: Gareth Copley
7. Roberts among Rams' five-player haul
Derby County have one of their busiest January transfer deadline days yet, bringing in a total of five new players on loan. The pick of the bunch were Man Utd defender Teden Mengi, and Man City winger Patrick Roberts. (Club website) Photo: Stu Forster
8. Preston beat Owls and Rovers to key targets
Preston North End strengthened their side with three new additions. Among the new recruits was Stoke's Liam Lindsay, who had also been linked with Sheffield Wednesday, and Ben Gordon, who was on Blackburn's radar. (Club website) Photo: Nathan Stirk