The pair both took to Instagram to break the sad news, which they described as an "incredibly difficult decision" in a joint statement.
'Cool For The Summer' singer Demi and Wilmer celebrated their six year anniversary at the beginning of this year, but on Friday (3 June), they revealed that their relationship had broken down.
In the message, the duo had wrote: "After almost 6 loving and wonderful years together, we have decided to end our relationship.
"This was an incredibly difficult decision for both of us, but we have realised more than anything that we are better as best friends. We will always be supportive of one another."
It went on to add: "Thank you to everyone who had offered us kindness and support over the years.
"With only love, Wilmer & Demi".
Last year, Demi was took to Instagram to deny rumours that the pair were engaged, but had revealed that she was "so in love, it's ridiculous."
In the past, Demi has been no stranger to gushing about her relationship with the That '70s Show actor. The former Disney star told fans she "still gets butterflies" when thinking about her long-term beau and described him as "magical".
The 23-year-old has also always cited Wilmer as the one who supported her through her struggles with addiction, bulimia and self-harm, which resulted in Demi entering rehab in 2010.
When she marked three years of sobriety in 2015, the U.S star thanked her beau in a touching tribute, writing :"I wish I could put into words how grateful I am for this man right here. But my love has grown to a level that words could never possibly express how much this man completes me.
"He's loved me the way I never thought I deserved to be loved and with this day marking my 3rd year sober... After sharing my ups, putting up with my downs and supporting my recovery... he still never takes credit and I want the world to know how incredible his soul is.
"I really wouldn't be alive today without him. I love you Wilmer."