The new arrangement extends Mattson's previous contract by a year, with an option for another season after next season draws to a close.
Mattson is currently second in the championship, 43 points behind Franklin F1's Nathaniel Powers.
Jean Mattson: "I'm very happy to be driving for KLM Team 23 next year. This year I have learned that it's crucial to have the right people working around you, and I feel very comfortable within this team.
"I'm confident the team can give me a car that is as good as the T23/1 next year. I would like to thank Andae d'Hazorme for his trust in my abilities and hopefully we can make Team 23 even more successful than it is already."