The South Coast side did not disclose the financial details of either of the deals, but British media reported that Brereton Diaz's move from LaLiga club Villarreal is worth around seven million pounds.
Born in Stoke-on-Trent, Brereton Diaz qualified to represent Chile through his mother and has been capped 33 times. A product of Nottingham Forest's academy, he joined Villarreal last year before moving on loan to Sheffield United in January.
"He (Brereton Diaz) is a strong, powerful runner and a confident finisher with a good goalscoring record, including last season's spell in the Premier League in which he adapted to the level extremely quickly." Southampton manager Russell Martin said in a statement.
Meanwhile, Matsuki will spend the 2024/25 season on loan at Turkish Super Lig club Goztepe.
Matsuki has represented Japan at various youth levels and helped them win the 2024 AFC Under-23 Asian Cup. He was also named the AFC Youth Player of the Year last year.
Southampton will begin their Premier League campaign away against Newcastle United on August 17th.
