The 27-year-old had been available for a short period of time following his release by La Liga outfit Almeria.
He has now agreed a contract at the Stadium of Light until the end of the season.
Ustari started his career with Independiente in his homeland, before spending five years with Getafe. Since then, he has turned out for Boca Juniors.
He was also part of the Argentina squad that won gold at the 2008 Olympic Games in Beijing.