Mells, 18, left Australia at the age of 12 to sign for Chelsea’s youth team and later moved to fellow English Premier League outfit, Southampton, when he was 14.
The attacking midfielder now returns to Australia in search of game time.
"It's a great opportunity for me to finally play some senior football and the club have a great philosophy especially with the Spanish influence through the coaching staff," Mells said.
“When I spoke to (coach) Josep (Gombau) it was clear that he promotes a very Spanish-like style, which is the type of game that suits me and it was one of the main reasons why I chose to come to Adelaide.
“It’s a very strong team and I’m honoured that the club has given me a place in the team for this season and I’m looking forward to having another successful season like they did last season.
“I want to make as many appearances as I can in the first-team and I want to have a big impact on the squad.
"Now is the right time for me to start stepping into senior football and I feel Adelaide is the best place for me to do that.”
During his time overseas, Mells earned himself national youth team call-ups from both Australia and Greece, and represented Australia U-17, as well as Greece U-17 in UEFA European championship qualifiers.
