The 33-year-old central midfielder has joined the Bulls on a one-season deal and is the club's third international player.
Benat has played for Spain four times and arrives at Macarthur after a lengthy stint in La Liga with Athletic Bilbao, where he played in the Champions League and Europa League.
All of Benat's international appearances came during 2012 when he was playing for Real Betis.
"Having played on some of football's greatest stages including the Champions and Europa Leagues against some of the biggest names in the world, a player of his calibre does not become available too often," Macarthur head coach Ante Milicic said.
"He brings a wealth of experience from the top leagues in Spain and will be extremely beneficial for the younger players in the squad to learn off."
Benat will link up with his new teammates after completing a mandatory 14-day quarantine period following his arrival in Australia.