With a big cheer from friends and family gathered behind the camera, Deacon shakes their hands and accepts his certificate – tassel moved to the other side — and lets out a great big "Yeah!" before throwing his cap in the air.
Witherspoon proudly watched her son graduate — and then fished the cap out of the pool.
She posted her own tribute to Deacon on Instagram congratulating him on his graduation.
"So proud of the thoughtful, talented, kind & generous young man you have become," the post read. "You light up our hearts and make us proud everyday."
The certified "valedriptorian" shared his own pictures to Instagram as well, including a close-up of his diploma, signed by his parents.
Deacon is the second child of Witherspoon and Phillippe, who also have a 22-year-old daughter together, Ava, pursuing a career in both modelling and acting.
Despite their split in 2006, Witherspoon and Phillippe appear to have maintained a friendly relationship.
Witherspoon is currently married to Hollywood agent Jim Toth, with whom she shares her nine-year-old son Tennessee.