"They were set up by Julianne," a source told Life & Style, adding that the couple has been dating for months.
"It got serious really fast and they're inseparable. They're already saying 'I love you.'"
Derek and Dobrev, 24, were planning to make their relationship public on Sunday night at the 65th Annual Primetime Emmy Awards -- where he ended up winning the Outstanding Choreography award -- but the actress had some last minute business to take care of instead.
After Derek accepted the honor, he reportedly went backstage and video chatted with Dobrev right away from Julianne's phone to tell her the news.
"Nina was on her way to the airport while Derek was making the press rounds," an eyewitness told the magazine. "He was holding up his Emmy to her and she was so excited for him!"
Added the source, "Now they're talking about having her go to DWTS to support him."
"I've been a relationship girl and I've been in long-term relationships my whole life until this point, so I think I'm looking forward to spending some time with myself and finding myself again," Dobrev told Us last month.
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"I'm just discovering different sides of me and growing and then I'll figure out what I like in a man when I see it."