"It baffles me why rumors are made up about who I am dating when I've always been upfront and open about my personal life," she tweeted.
Thorne and Posey were spotted shopping together in West Hollywood and the rumors starting swirling.
"I don't really like dating a bunch of people," Thorne said. "I kinda don't get it. I kinda don't understand it. I like to kinda just be with one person if that's OK and just stick to them."
Thorne and her ex-boyfriend Tristian Klier broke up in August after almost three years of dating.
In March, Thorne was linked to Pamela Anderson's son Brandon Lee but E! News exclusively confirmed that now she is "just dating."
"Brandon and I reconnected recently," Thorne said on The View. "We met during a breakup with my ex, he's really sweet."
The Duff actress posted a picture of herself lying in her friends lap with the caption, "I'm going to start my own rumor and tell everyone I'm dating @Alexa_Yarnell because she's hot."
— bella thorne (@bellathorne) April 29, 2015
We love our girl Bella for shutting down those rumors!