The 27-year-old moved to La Liga from Boca Juniors at the start of the campaign, but did not make a single first-team appearance before departing.
It has been revealed that Sunderland have secured his services until the end of the season, but Ustari has taken the time to praise his former club.
"Although I've not had many games, I've been very happy here these last six months. Almeria is a marvellous city to live in and I won't forget it. I'll leave behind a lot of friends at the club," he told the Spanish club's official radio station.
"I want to thank Almeria for everything they've offered me, the club, my teammates, the coaching staff, the fans, everyone. I hope and I'm confident that the team will achieve permanence in La Liga, with plenty of games to spare."
Ustari started his career with Independiente.