The 21-year-old actor asserted as much Monday on Twitter after being photographed with another woman in Cabo San Lucas, Mexico.
Schwarzenegger is a student at University of Southern California and was on spring break when he was spotted cozying up to a bikini-clad sorority girl and taking body shots off her midriff.
"Omg. It's one of my best friends girlfriend..." Schwarzenegger tweeted after the pictures surfaced online. "Would NEVER do anything against my GF..... Girls have guy friends and guys have friends that are girls."
Schwarzenegger and Cyrus have been linked since November 2014, and attended dinner with the actor's mother, Maria Shriver, in February. Schwarzenegger recently shared a photo of himself with Cyrus for Valentine's Day, and captioned the picture, "Luckiest guy in the world."