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.
1. Tykes land USA international
Barnsley sealed a late loan move for Orlando City striker Daryl Dike. The 20-year-old made his USA senior debut last month, in a friendly match against Trinidad and Tobago. (Club website) Photo: Emilee Chinn
2. Hatters snap up Ince
Luton Town completed a couple coup on deadline day, bringing in Stoke City's £12m man Tom Ince on loan, and securing Walsall striker Elijah Adebayo on a permanent deal. (Club website) Photo: Justin Setterfield
3. Ex-Boro stopper joins Terriers
Huddersfield Town brought in former Basel and Middlesbrough goalkeeper Jayson Leutwiler. He left Fleetwood Town last month, and was briefly a free agent after his contract with the club expired. (Club website) Photo: Lewis Storey
4. Forest move for Murray
Veteran striker Glenn Murray left Brighton to join Nottingham Forest until the end of the season. He had been on a loan spell with Watford, but had only started one game this season prior to his switch to Forest. (Club website) Photo: Richard Heathcote