The separation was supposedly very sudden and left the 22-year-old star inconsolable.
The couple first went public with their relationship at the premiere of her movie 'The House Bunny' in 2008 and had appeared as a fixture on the Hollywood scene ever since.
A source told People: "They decided it wasn't working. They're moving on."
While a friend of the duo added to Life and Style magazine: "She is very upset.
"She went to Las Vegas this past weekend with a few friends to try to cheer up and have fun, but she's very sensitive about the break-up."
Rumer was spotted partying last weekend at top nightspot Tao in Las Vegas with 'Glee' star Mark Salling and his actor friend Chord Overstreet in a bid to get over Micah.
And at one point, the pretty brunette and Chord flirted on the dancefloor.
An onlooker added to People: "Rumer was bouncing around the club having a great time just dancing. Chord was flirtatious with Rumer. They had a lot of fun."
US soap star Micah - who starred in hit shows 'All My Children' and 'One Life To Live' - appeared to have been accepted by Rumer's family, often appearing with his girlfriend and her mother Demi and her stepfather Ashton Kutcher on the red carpet at movie premieres.