The Vampire Diaries star met Derek through professional dancer Julianne Hough, and fell for her close friend's older brother after "hanging out" with the siblings.
While Nina and Derek have previously kept quiet about their romance, it seems the lovebirds are ready to go public with their relationship.
The 24-year-old actress posted a photo of Derek holding his Emmy award earlier this week, and affectionately wrote, "@Derekhough !!!!!!!! #EMMYWINNER I can't express in 140 characters how unbelievably excited+incredibly proud I am of…"
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Nina clearly had Derek on her mind as she uploaded another snapshot later that day, with the caption,"@derekhough Those of us touched by your light got to watch you brighten up the rest of the world. Keep shining! …"
The Dancing with the Stars professional responded to Nina's tweet, writing, "[You're] the best babe."
The couple reportedly started dating in August, but are already quite serious about each other.
"Derek is smitten with Nina," an insider told Us Weekly. "He said he hasn't felt this way for a girl in a long time. Everyone approves."
Nina, who split from her boyfriend of three years Ian Somerhalder in May, had previously said she wasn't looking for a new romance.
"I've been a relationship girl and I've been in long-term relationships my whole life until this point," said Nina. "So I think I'm looking forward to spending some time with myself and finding myself again.
"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."
Derek, meanwhile, has previously dated Cheryl Cole, actress Shannon Elizabeth and reality TV star Lauren Conrad.