Romario, a former World Player of the Year and the second highest goal scorer of all time is scheduled to make his first appearance for Adelaide on November 25 away to the Central Coast Mariners.
The 40-year-old, who scored 55 goals in 74 internationals for Brazil, will go to Adelaide after playing in the First Division of the USA's United Soccer League with Miami FC this year.
"He's got an art you can't coach, you're born with it," Adelaide United coach John Kosmina said.
"The skill and grace he's got doesn't dissipate with time while he's still practising. He's never been renowned for his work rate but his work rate is around the edge of the box and that's all that counts.
"He's won World Cups, he's played for some of the best clubs in the world. There will be a long term benefit in this for everybody and that's the important aspect of the deal."
The Brazilian forward has won three Dutch championships with PSV Eindhoven and one Spanish title with Barcelona, with his arrival in South Australia also bringing an exhibition match against Miami FC in Adelaide on October 17 as part of the deal.
Romario will also be involved in Adelaide's matches against Melbourne Victory on December 1, New Zealand Knights on December 10 and Newcastle Jets on December 15, all at the club's home ground Hindmarsh Stadium.
The announcement of Romario's guest appearances for Adelaide United follows fellow A-League club Sydney FC's unveiling of Italian Benito Carbone on a similar four-match deal which commences on Monday for the current A-League champions.