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.
9. Boro beat Derby to winger
Middlesbrough beat off competition from Derby County to sign ex-Cardiff City winger Nathaniel Otis Mendez-Laing. He previously played under Neil Warnock when the veteran manager was in charge of the Bluebirds. (Club website) Photo: Alex Livesey
10. Millwall seal Evans deal
Millwall landed ex-Man City starlet George Evans on a permanent move from Derby County. The versatile 26-year-old is capable of playing as both a defender and a midfielder. (Club website) Photo: Stephen Pond
11. Cherries set for Long wait
Bournemouth are waiting for EFL approval to confirm their late loan swoop for Southampton forward Shane Long. The Republic of Ireland international has started just once for the Saints this season. (Club website) Photo: Naomi Baker