The 29-year-old attacker will link up with the Sky Blues as they begin life under Steve Corica, who has succeeded new Socceroos boss Graham Arnold as the team's head coach.

Capped six times by the Netherlands, De Jong won the Eredivisie title four times with Ajax, but failed to convince during a three-year stay at Newcastle United in the Premier League from 2014 to 2017.

He spent 2016-2017 on loan at PSV, before returning to Ajax last year.

De Jong will hope to help Sydney recover from the disappointment of losing last year's Grand Final to Melbourne Victory after dominating the regular season. 

"I'm very excited to be representing Sydney FC this season and I'm looking forward to my time in the A-League," he said. 

"I see a lot of similarities between Sydney FC and Ajax, they're both leaders in their league with a reputation for silverware and I want to uphold that here.

"I want nothing more than success and I'll help the squad in any way to achieve our goals of winning all of the trophies we compete for this season."