De Jong left Turkish side Galatasaray by mutual consent on Friday, and his move to Mainz was confirmed shortly after.
The 33-year-old, who has previously represented Manchester City, has joined the German outfit on a contract until the end of the 2017-18 campaign.
"I like Mainz's story, the club has established itself in the Bundesliga off its own back," De Jong told the club's official website.
"I want to help the team and the club achieve its aims this season. I'm really pleased to be back in the Bundesliga, one of Europe's top leagues."
De Jong will join a team that are currently 15th in the Bundesliga table - just two points clear of 16th-placed Werder Bremen after 17 matches of the 2017-18 campaign.