"Six years ago I was lucky enough to marry this wonderful man. He makes me laugh everyday and supports me in everything I do. Happy anniversary, JT! Here's to many more!" she wrote next to a picture of the happy couple watching the sunset on the beach.
The "Big Little lies" star married Toth in 2011 in California. They are parents to son Tennessee, 4.
It's been a big week in other ways for Witherspoon, as she celebrated her 41st birthday.
"On top of the world from all of your sweet birthday wishes!" she wrote in an earlier post. "There’s nothing better than celebrating my birthday with my wonderful kids!" she added in another Insta pic.
Phillippe and Witherspoon have both spoken openly about how well their co-parenting works for them.
"You have to get to that point as a divorced parent, as any parent, where you’re not putting yourself first," Phillippe told Entertainment Tonight last year.