Former club captain McKain was a key member of the team which reached the A-League finals in impressive style during Josep Gombau's first season in charge.
Along with playmaker Marcelo Carrusca, the 31-year-old was due to come out of contract at the end of the campaign, with the Reds' Spanish coach confirming in February offers had been made to both players.
And while Argentine Carrusca agreed a new deal later that month, there was no word on McKain's future until Thursday, when the 16-cap former Socceroo announced he would not be staying.
Adelaide, who re-signed central defender Nigel Boogard last month, are yet to comment on McKain's situation.
Dissapointed to be leaving Adelaide United. Hoped and wanted to stay but not to be. A new adventure awaits somewhere.— Jonny McKain (@JonnyMcKain) May 15, 2014