Cirio's contract was set to end at the end of this season but both parties were delighted to extend his stay in the city of churches.
Since joining the Reds before the 2013-2014 season, the winger/attacker has scored 18 times in 42 games including the winner in the inaugural FFA Cup final win over Perth Glory.
"There are a lot of reasons behind why I have decided to re-sign with Adelaide United," Cirio - affectionately known as 'El Eanque' (the tank) - said.
"I am happy here in Adelaide, the team is playing very good, and we're in a good position in the A-League.
"The most important reason is because I want to win things here with United because I am really appreciative of everybody, from the coaches and staff to the fans as well.
"From my first day I have felt like I have been at home, so I feel I should return all these great things.
"I always look forward to our next game because I want to play, I want to score, and I want to help the team as much as possible.
"These are the only things that I want – to play and to enjoy it with my team-mates.
"Josep is also giving me a lot of confidence and I'm playing a lot too. That is an important reason as well to me signing on with the club."
Coach Josep Gombau said the new deal for 29 year-old Cirio was a significant boost for the Reds leading into the second half of the season.
"I am so happy that Cirio has extended his contract. Sergio is the type of player that suits the style we are playing and also a very good person in the group," said Gombau said.
"These qualities are very important to the club and bring a lot of value to the team.
"He is now at the age where he should be able to play his best football and we are confident that over the next two years he can be one of the important players in our team."
