"Beachers Madhouse," the Keeping Up With the Kardashians star captioned an Instagram clip, which showed her and Montana dancing and smiling to his hit song "Pop That," as a costumed Big Bird and Elmo danced on stage, with Cookie Monster showing off his moves in the crowd.
"Khloé and French came in together with about 10 friends, and no other celebs were with them," a source told E! News exclusively. "They only stayed an hour or so...Their table was opposite Nick and Joe Jonas."
French and Khloé's table sipped on Yacht Club Vodka and Casamigos Tequila, and then they left at around 2 a.m.
The duo was "whispering and hugging a lot" throughout the night, the source added.
This is just the latest sighting of Kardashian and Montana, who have been spending quite a bit of time together in recent weeks.
Most recently, the pair was spotted stepping out for a dinner date Monday at Casa Vega restaurant in Hollywood Monday, arriving in the E! star's car with Montana in the passenger seat and their pal in the back.
-Additional reporting by Sara Kitnick