Jorginho, an Italian midfielder, has been acquired by Arsenal from Chelsea for £12 million.
The 31-year-old was in the final six months of his deal at Stamford Bridge when he transferred from Napoli to Chelsea in 2018.
Tuesday, after passing a physical exam, he signed an 18-month contract with a one-year extension option.
After having two bids for Ecuadorian midfielder Moises Caicedo rejected by Brighton, the Gunners switched their focus to Jorginho.
Brighton said Caicedo was not for sale and he looks to set to remain at the club after the 11pm deadline.
Jorginho becomes Arsenal’s third signing of the January window, the club having previously bought Poland defender Jakub Kiwior from Spezia for 20m euros (£17.6m) and Belgium striker Leandro Trossard, who moved from Brighton for £21m.
Mikel Arteta’s side are five points clear at the top of the Premier League with a game in hand on the sides immediately below them.
Jorginho has scored 29 goals in 213 appearances for Chelsea, winning the Champions League, Europa League, Uefa Super Cup and Fifa Club World Cup during his time at at the club.
