Sunderland have signed Manchester United goalkeeper Nathan Bishop for an undisclosed fee on a three-year deal.
The 23-year-old joined United from Southend in January 2020 but never made a competitive appearance for the Premier League side.
He could make his Black Cats debut against Ipswich Town on Sunday.
Bishop was involved with a collision with Wrexham striker Paul Mullin during a pre-season friendly that left the latter with a punctured lung.
Sunderland skipper Luke O'Nien has signed a new three-year deal to stay on Wearside.
The 28-year-old defender, who joined the Black Cats from Wycombe in 2018, has made 225 appearances in all competitions, scoring 18 goals.
He was part of the Sunderland side that won promotion back to the Championship via the play-offs in 2022.
"I've got so many people to thank for the journey so far. I've loved my five years here," said O'Nien.
"I didn't want it to end and I can't wait to continue my time here and create more incredible memories with some great people."
O'Nien, who began his career with Watford before moving to Wycombe, also helped the Black Cats to EFL Trophy success in 2021.
Chelsea have loaned striker Mason Burstow to Championship side Sunderland for the rest of the season.
The 20-year-old forward joined the Blues from Charlton in February 2022, and made his Premier League debut in August against West Ham.
He featured in three games this season, including a first Blues start in the Carabao Cup in midweek.
"It's a huge club with a great head coach [Tony Mowbray] and big expectations," Burstow said.
"I'm here to bring goals to the team and keep supporters on the edge of their seats. I can't wait to walk out at the Stadium of Light for the first time after the international break."
Former Welling player Burstow will not be registered in time for Saturday's lunchtime kick-off against Southampton.
Sunderland have signed defender Timothee Pembele, midfielder Adil Aouchiche and strikers Mason Burstow and Nazariy Rusyn.
Burstow, 20, has joined on loan from Premier League side Chelsea, having played three games for the first-team.
Rusyn, 24, joins from Zorya Lugansk on a four-year deal for an undisclosed fee, after 21 goals in 49 games.
Pembele, 20, joins from Paris St-Germain, while Aouchiche, 21, arrives from Lorient - both on five-year deals.
Defender Pembele played 15 first-team games for the French champions, and moves to the Black Cats for an undisclosed fee.
Former team-mate Aouchiche left PSG for Saint Etienne, before moving to Lorient and has played 86 first-team games in total.
Burstow was at Charlton before moving to Stamford Bridge, where he made his first-team breakthrough earlier this season against West Ham.
Ukranian forward Rusyn also had spells at Dynamo Kyiv and Dnipro, as well as Polish side Legia Warsaw.
Burstow said of his season-long switch to the Stadium of Light: "It's a huge club with a great head coach [Tony Mowbray] and big expectations.
"I'm here to bring goals to the team and keep supporters on the edge of their seats. I can't wait to walk out at the Stadium of Light for the first time after the international break."
Former Welling player Burstow will not be registered in time for Saturday's lunchtime kick-off against Southampton.
Rusyn will travel to Sunderland once the paperwork is completed.
Sunderland goalkeeper Anthony Patterson has signed a new contract aimed at keeping him at the club until 2028.
The England Under-21 international has become a regular for the Black Cats since the 2021-22 season, when he helped them to win promotion to the Championship from League One.
Patterson played in all 46 of Sunderland's league fixtures last term.
Sporting director Kristjaan Speakman said Patterson's long-term deal was a "real statement" by the club.
"He's establishing himself as one of the best young English goalkeepers," Speakman added.
"We have confidence in him and his ability to further improve, and equally he has confidence in us, which I feel is important.
"As an academy graduate, we couldn't be happier for Anthony and his journey so far is a testament to the work undertaken by him and so many staff over a significant period of time."
Patterson, who has helped the Black Cats to four wins and one draw in their last five Championship matches, said "You can see the trajectory the club is on and it's an exciting time to be here.
"It's obviously an absolute dream to come through the academy to where I am now. When you first join the club at the age of 10, you don't ever think too much about actually playing for Sunderland."
Sunderland winger Patrick Roberts has agreed a new long-term contract at the Stadium of Light.
The 26-year-old has agreed a deal to stay on Wearside until 2026 with the club option of a further year.
The former Fulham and Man City man has made 79 appearances since joining the Black Cats in January 2022.
He told the club website: "I can't wait for the next few years together and hopefully we'll have more success along the way.
"I've enjoyed every moment of being here and I'm delighted to get the deal done. The supporters have been great with me and it's been an enjoyable journey so far, and long may that continue. Sunderland is a huge club with huge expectations."
Sporting Director Kristjaan Speakman added: "Patrick has been at the heart of our progress throughout the last two seasons, and he has grown into an important member of our group. He possesses some elite qualities and at the age of 26, we firmly believe he can continue to develop, improve and evolve.
"We are really happy that he has signed a new deal and his commitment to the club not only signifies his belief in our progression, but also our constant pursuit to improve."
Sunderland have signed defender Leo Hjelde from Championship rivals Leeds United on a four-and-a-half-year deal for an undisclosed fee.
The 20-year-old centre-back has been limited to just three appearances for Leeds this season, and eight in total.
Hjelde joined Leeds from Celtic, having previously been at Norwegian side Rosenborg, and has been on loan to Rotherham and Ross County.
"I'm a ball-playing centre-back, or left-back," Hjelde said.
"I like to get up and down the pitch. I've been at some big clubs previously and this has helped me develop as a player and as a person, but I'm now ready to push on and take the next step."
The son of former Nottingham Forest defender Jon Olav Hjelde, Leo was born in Nottingham and has spent a large part of his career in Britain, as well as representing Norway Under-21s.
"I spoke to my dad, who showed me the size of the club and the support," Hjelde added. "I knew it was a move I simply couldn't turn down."
Meanwhile, Black Cats forward Jewison Bennette has joined Greek top-flight side Aris on loan until the end of the season.
The 19-year-old Costa Rica international has made two appearances for Sunderland this season.
Sunderland have signed winger Romaine Mundle from Standard Liege on a four-and-a-half year deal, while Barnsley midfielder Callum Styles joins on loan.
Mundle, 20, left Tottenham to join the Belgian top flight side last summer and has played six league games for them.
He never made a senior appearance during his time at Spurs.
Meanwhile, Sunderland have the option to make their deal for 23-year-old Hungary international Styles permanent at the end of the season.
Styles, who has made 151 appearances for Barnsley, told the Tykes during the transfer window that he wanted a winter move to play at a higher level this season.
"We respected Callum's wishes, which we felt was beneficial for all parties as we look to push on and achieve our goals for this season at Barnsley," said Barnsley's interim director of football Bobby Hassell.
Both players could make their debut against local rivals Middlesbrough on Sunday.
Birmingham sign Sunderland midfielder Pritchard
Sunderland started transfer deadline day by bringing in Mundle for an undisclosed fee, with sporting director Kristjaan Speakman describing the Londoner as "highly sought after".
"We didn't anticipate him being available on a permanent basis, but when we sensed an opportunity, we moved quickly and decisively to make it happen - our ways of working and structure are a real strength in these moments," Speakman added.
Sunderland defender Aji Alese has signed a new deal to remain with the Championship side until the end of the 2026-27 season.
The 23-year-old moved to Wearside from West Ham United in the summer of 2022 and has made a total of 27 appearances.
Alese has been limited to just three appearances for the Black Cats this term, however, because of injury.
"I've had some great experiences so far here and some which I can grow and improve from moving forward," he said.
"From the moment I joined I felt the warmth of the supporters and I can't wait to get back out there playing in front of them."
Sporting director Kristjaan Speakman added: "He's performed at a high level for us and demonstrated the potential to develop further, so we see him as an important member of our squad.
"It's been a difficult 12 months for him due to injury, but his return to action is imminent and his personality has ensured that he has contributed to the group throughout his rehabilitation."