The 27-year-old had his contract at Almeria ended by mutual consent earlier on Tuesday, leaving him free to complete a deal at the Stadium of Light.
Ustari had only been with the Liga outfit since the close-season after joining from Boca Juniors, but failed to break into the first-team picture at Estadio de los Juego Mediterraneos.
Manager Gus Poyet has been keen to add to his goalkeeping options after Keiren Westwood suffered a shoulder injury in November that could potentially keep him out of action for the rest of the season.
Ustari will now provide competition for Vito Mannone, who has made 13 appearances in the Premier League this season keeping five clean sheets.
He becomes the third signing of the January transfer window for Poyet, who has already bolstered his ranks with the signing of Marcos Alonso and Santiago Vergini, who arrived from Fiorentina and Estudiantes respectively.
Ustari has two caps for the Argentina national side, and appeared for his country at the 2008 Beijing Olympic Games.
