Guillermo advised the club of his decision Wednesday afternoon.

The Spanish boss joined the club as its Technical Director for the 2014/15 A-League campaign before taking over the reins as head coach in 2015.

In his first senior coaching role, the Barcelona legend led the Reds to an historic A-League 2015/16 Premiership and Championship double.

“The club would like to thank Guillermo for his diligence and hard work during his time with us, as well as guiding us to our long-awaited Championship in front of a packed Adelaide Oval last year,” the club said in a statement.

“Adelaide United has already commenced a recruitment process to appoint a new head coach and will ensure new coaching personnel are appointed by mid-June, in time for the club’s pre-season preparations.”