Reese Witherspoon has thrown a surprise graduation party for her two eldest children.

The actress took to Instagram on Sunday (10Jun18) to share details about the fun-filled bash she held at her home to commemorate her 18-year-old daughter Ava's completion of high school and the end of her 14-year-old son Deacon's time in junior high.

Pictures taken at the intimate gathering showed that Reese, who also shares five-year-old son Tennessee with her husband Jim Toth, had gone all out for the celebration, with one photograph showing that she had the message "You Did It" made from colourful balloons hung upon a garden hedge.

The graduates, who she shares with her ex-husband Ryan Phillippe, also posed by their swimming pool while wearing black gowns and mortarboard hats, before the Legally Blonde star filmed the pair throwing the caps into the air.

Ava and Deacon were also treated to a delicious spread of food, which included a huge plate full of colourful vegetables, as well as a large jug of red punch, which read "Cheers Y'all!" on the glass.

Reese's eldest daughter took to her own social media account to celebrate the milestone too.

"I graduated high school! (here's what I looked like at the beginning of it)," Ava wrote, accompanied by a picture of her from her younger years, in which she rocked long, bright pink hair.

In addition to the achievements of her children, Reese had further cause to celebrate, as just last week the Oscar winner announced plans for a third Legally Blonde movie.

"It's true... #LegallyBlonde3," the 42-year-old star posted alongside a video of her sporting a pink bikini while lying on a blue lounger in a pool, a reference to the costume she wore in her admission video for Harvard University in the first movie.