Sunderland have agreed terms with Central Coast Mariners for the transfer of Nectarios Triantis.
The 20-year-old central defender is expected to join the Black Cats for an initial fee of £300,000.
Triantis is an Australia Under-20 international who is expected to move up to their under-23 squad this summer.
Triantis played 25 times for the Mariners and started as they beat Melbourne City in the A-League grand final at the weekend.
Sunderland are looking to reinforce their squad after reaching the play-offs in their first season back in the Championship for five years.
They suffered problems with injuries to central defenders at key moments of the season with Danny Batth, Dan Ballard and Aji Alese all missing for lengthy periods.
"The opportunity to join such a prestigious club is an honour and I'm really looking forward to heading to England and meeting my new teammates later this month," Triantis said.
"I'm ready for the challenge ahead and can't wait to get started. I would like to thank Nick Montgomery and the Central Coast Mariners for giving me the platform that led to this exciting next chapter in my life."
The transfer is subject to international clearance and cannot be completed until the transfer window opens next Wednesday.
Defender Trai Hume has signed a new four-year contract with Sunderland, to run to the summer of 2027.
The 21-year-old Northern Ireland international scored two goals in 34 games for the Black Cats last season, including the play-off semi-finals.
Hume, who previously played for Linfield and Ballymena, established himself as a first-team regular under boss Tony Mowbray last season.
"I'm still young and I'm certainly looking to play games," Hume said.
"Hopefully we can have another good season but there is lots of work over the summer."
Sporting director Kristjaan Speakman added: "Trai was very patient in his first twelve months and worked tirelessly to refine his game.
"Since the new year he's gone from strength to strength, grown in confidence and his performances merit an improved and extended contract."
Sunderland have signed Birmingham midfielder Jobe Bellingham for an undisclosed fee, on the same day his older brother Jude completed a move to Real Madrid from Borussia Dortmund.
Like England international Jude, Jobe, 17, came through the Blues' academy and made his senior debut aged 16.
He made 23 appearances in all competitions last season.
Championship side Sunderland say he will complete the transfer when he returns from England Under-18 duty.
"I'm delighted to sign for Sunderland and I can't wait to get started," he told the club website.
"The opportunity for young players to move here and progress is clear, so I believe this is the perfect club for me to continue my development."
Tony Mowbray's side finished sixth in the Championship last season before losing to Luton in the play-offs.
The Black Cats have not disclosed the length of contract Bellingham will be signing.
Sunderland defender Dennis Cirkin has signed a new three-year contract with the Championship club.
He made 28 league appearances last season, scoring five goals, and has agreed to extend his contract with the Black Cats until 2026.
The 21-year-old moved to Wearside in August 2021 from Tottenham Hotspur.
"This is one of the best places to be for young players at the moment. I think this is the perfect place for me," said the left back.
Cirkin has made a total of 62 appearances for Sunderland and helped his team to a sixth-placed finish in the Championship last season, before they lost to Luton in the play-off semi-finals.
The England Under-20 international joins fellow defender Trai Hume in signing a new deal at the club.
Sunderland have signed Benfica striker Luis Hemir Silva Semedo for an undisclosed fee on a five-year deal.
The 19-year-old featured for the club's B team in the Portuguese second tier in the past two seasons.
"This is a big move for me and I am very excited to be here and meet my new team-mates," the Portugal Under-20 international told the club website.
Tony Mowbray's men finished sixth in the Championship last season before losing to Luton Town in the play-offs.
Sunderland have signed Central Coast Mariners defender Nectarios Triantis on a four-year deal.
The 20-year-old centre-half has joined the Black Cats for an initial fee in the region of £300,000.
Triantis is an Australia Under-20 international who is expected to move up to their under-23 squad this summer.
Triantis played 26 times for the Mariners and started as they beat Melbourne City to win the A-League Grand Final earlier this month.
Sunderland are reinforcing their squad after reaching the play-offs in their first season back in the Championship for five years.
They suffered problems with injuries to central defenders at key moments of the season with Danny Batth, Dan Ballard and Aji Alese all missing for lengthy periods.
"When a massive club like Sunderland comes in, you can never say no," Triantis said.
"It was a dream to have won the league with Central Coast Mariners, but now I hope to enjoy similar success with Sunderland."
Sporting director Kristjaan Speakman added: "Nectar' has progressed rapidly over the last 12 months - at both club and international level - and we are delighted that he selected Sunderland as his next step."
Sunderland have completed the signing of PSV Eindhoven defender Jenson Seelt for an undisclosed fee.
The 20-year-old, who has signed a five-year contract, has made one Eredivisie appearance, against PEC Zwolle in May 2022, and featured in matchday squads last season.
Seelt has played for reserve side Jong PSV in the second tier on 63 occasions, 33 of which came last term.
He came through the Eindhoven club's academy, which he joined in 2017.
"Jenson is a player that we have been tracking for some time and he's gained invaluable experience at senior level over the past two seasons, regularly playing in the Dutch second tier and being part of PSV's senior matchday squad," Sunderland sporting director Kristjaan Speakman told the Sunderland club website.
"Naturally comfortable in possession, he also possesses some great defensive attributes that will help him adapt to our playing identity and Championship football."
Sunderland defender Bailey Wright has left by mutual consent in order to "pursue an opportunity overseas", the club have announced.
Wright joined Sunderland from Bristol City, initially on loan, before signing permanently in 2020.
The Australia international, 30, made a total of 107 appearances, helping them win promotion from League One and the EFL Trophy in 2020-21.
"I've loved my time and I've had some incredible experiences," he said.
"I look back on what has been an incredible journey - the highs of Wembley wins to the lows that came before - and I wouldn't change a thing about my experience.
"You want to try and leave an impression and ensure the club is in a better place than when you found it, and I feel like I've been part of a group that has done that."
underland have signed former Blackburn attacking-midfielder Bradley Dack on a one-year deal.
The 29-year-old reunites with Black Cats manager Tony Mowbray, who he played under during his time at Rovers.
Dack scored 57 goals in 173 appearances in a six-year spell with Rovers despite missing a large chunk of games through separate cruciate ligament injuries.
"To play in front of 40,000 every week at the Stadium of Light was a huge pull for me," Dack told the club website.
"I hope my experience and style of play can add something different to the group, and I can't wait to pull on the shirt for the first time."
The deal includes a club option for an additional year.
Greenwich-born Dack had spells at Wimbledon and Charlton before joining Gillingham where he played 185 games, scoring 38 goals.
His form with the Gills attracted Rovers' attention, prompting a move to the north-west in June 2017 that brought promotion to the Championship in his first season.
He scored 18 goals in that triumph, followed that with 15 in the second-tier, and had nine goals before his first knee injury in 2019.
Last season he netted seven goals in 33 appearances before ending his six-season stay at Rovers this summer.
Sunderland have signed Sochaux striker Eliezer Mayenda for an undisclosed fee on a five-year deal.
The 18-year-old former Spain Under-17 international became the French side's youngest ever player when he made his debut aged 16 years and seven months in December 2021.
"I am very happy to be here because it is what I wanted. It is a historic club in England," he told the club website.
"I really wanted to take up the new challenge."