The couple, who got together after partnering for Strictly's 2010 series, revealed their happy news on Twitter yesterday (January 6).
"Sorry I've been a bit quiet, wanted to wish you all a very happy new year," Cacace wrote. "I've had a wonderful time and have some amazing news.
"All I need to say is @JimiMistry has made me the happiest girl in the world. Xxxx."
East is East actor Mistry responded on his own profile: "And you've made me the happiness man xxxxx."
The pair later thanked fans for their congratulatory messages, with Mistry adding: "[I] can't stop smiling."
However, their joy was scuppered somewhat when it was erroneously reported that Cacace had been engaged before to fellow Strictly pro Vincent Simone, who she was with before striking up a romance with another of her partners on the show, ex-EastEnders and Hustle actor Matt Di Angelo.
"Contrary to some reports this is my first ever engagement," she wrote. "It does make me laugh how incorrect information is copied from one article to another."
Cacace won the latest edition of Strictly Come Dancing last December with Olympic gymnast Louis Smith. She and Mistry were eliminated fifth in their series.