The couple - who have been dating for around two years - are set to tie the knot after Dax proposed over the festive period.
Kristen's representative said: "They got engaged over the holidays."
The 29-year-old actress debuted her stunning ring as she presented an award at last night's Grammys (31.01.10).
Kristen - who worked with Dax, 35, on romantic comedy 'When in Rome' - recently revealed her love life takes priority over her career.
She said: "I think your significant other should be way more important than your work, personally. I love working. But I don't know that I'd say I'm a workaholic because I also love maxin' and relaxin'."
She also gave her top relationship tip - communication.
She added: "I've learned the necessity for healthy communication. How important it is to talk about your deepest, darkest fears and feelings. Your partner should hear your most embarrassing insecurities, because once you say them out loud, you've created a safe zone.
"And a snuggle party can fix anything - it's true."