The 33-year-old has been allowed to leave Turkish giants Galatasaray to head back to Germany, where he previously played for Hamburg between 2006 and 2009.
I want to help the team and the club achieve its aims this season.
He then left for spells with Manchester City, AC Milan and LA Galaxy, before heading to Turkey in the summer of 2016.
De Jong has also won more than 80 international caps for the Netherlands, helping the Oranje reach the final of the 2010 World Cup, where they were beaten 1-0 by Spain.
And the midfield enforcer is relishing his latest challenge, telling his new club’s official website: “I like Mainz’s story, the club has established itself in the Bundesliga off its own back.
“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.”
Meanwhile, sporting director Rouven Schroder is delighted to have brought such an experienced performer to the Opel Arena, adding: “Nigel is a leader and a fighter, but has a cool head and is very professional.
“He’s going to help the team with his experience and give us a real boost ahead of the second half of the season. We’re really pleased to bring in a player of his quality.”