Last year, Goodwin signed a deal with Adelaide that tied him to the A-League Men club until 2025.
But the Reds confirmed on Friday that their skipper will be leaving to join a yet-to-be-revealed Saudi Pro League club.
It's not the first time Goodwin has left Adelaide.
The 31-year-old took up deal with Dutch outfit Sparta Rotterdam in 2016, and after returning home, he left Adelaide again in 2019 for stints with Saudi Arabian clubs Al-Wehda FC and Abha.
It's been reported Adelaide will receive a seven-figure transfer fee, with Gooodwin set to earn approximately $2.5 million per season at his new club.
Superstars such as Cristiano Ronaldo, Neymar and Karim Benzema currently ply their trade in the cashed-up Saudi competition.
Goodwin said it would be "life changing" for himself and his family.
"It was never really in my plans to leave Adelaide, but this opportunity presented itself and it was an offer that the club and I decided was best for both parties," Goodwin said in a statement.
"I'm a South Australia boy, it's a special place and it's where I have so many memories, and experiences, and I have made so many lifelong friends from the pitch to the grandstands.
"It's where I started my footballing journey, playing for this club and representing this state and its people has been a privilege.
"I will now have the chance to play alongside some of the best in the world and keep testing my ability, which is what drives me as a professional footballer."
In a video to fans, Goodwin added: "Hopefully it's not the last you'll see of me at Adelaide United."
The loss of Goodwin is a major blow to Adelaide's title hopes.
Goodwin starred at last year's World Cup, scoring against France, and won the Johnny Warren Medal last season as the A-League Men's best player.
He has scored 58 goals across his 205-game ALM career.