Khloe shared a photo on Snapchat from her workout with Tristan before they returned home and tested out a giant water slide for the bash. She followed up with a video of herself and Kris at the party.
"Happy Fourth of July!" the pair wish their fans.
Rob attended the fête with Dream, his 7-month-old daughter with ex-fiancée Blac Chyna. He posted a series of pictures with Dream on Instagram, including a snapshot of himself holding his baby girl in Khloe's pool.
"Happy 4th! Everyone be safe !!! Love," the 30-year-old reality star captioned the photo.
Khloe and Tristan, who plays basketball for the Cleveland Cavaliers, were first linked in October. Tristan helped surprise Khloe with a party ahead of her 33rd birthday in June, and dedicated a sweet message to the star on her actual birthday.
"Happy bday my love," he wrote June 27. "Let's continue to build more memories and cherish them with each other. God bless you, I love you."