Kate, 35, and Derek, 29, sparked romance rumors when they were spotted together as recently as this past weekend at the Nice Guy Restaurant in Los Angeles, according to Us Weekly. One eyewitness says the two ate dinner, drank and stayed past closing.
Kate and Derek have been friends for three years, and he's even given the super-fit actress dance lessons in the past.
"My heart is really in contemporary jazz, but lately I've been ballroom dancing with Derek Hough," she told Shape magazine in 2013. "It's been a real blast."
Just last month, the two attended Goldie Hawn's "Love In For Kids" benefit, where they were snapped having a friendly conversation. They two also posed for pictures together, Kate showing off her toned arms in a sleeveless white dress and Derek looking dapper in a dark suit.
This of course isn't the first in-demand Hollywood starlet Derek has been linked to. Derek has also been rumored to have romanced Lauren Conrad, Nina Dobrev and Nikki Reed in the past.
News broke of Kate and Matt's split on Monday, when his rep confirmed that the two have been separated "for some time now," after four years together. The two got engaged in April 2011, three months before Kate gave birth to their son, three-year-old Bingham Hawn Bellamy.
"They remain very close friends and committed co-parents," Bellamy's rep told ET.
In May, ET's Rocsi Diaz caught up with the How to Lose a Guy in 10 Days actress at the launch of her workout company Fabletics' Spring 2014 collection, who at the time said she was still planning on marrying Bellamy despite their long engagement.
"Matt and I -- not yet," she said when asked about the possibility of an upcoming wedding between the two. "I think we will for the kids at some point, but it's still just not something we're focused on planning."