The "I Love Me" singer's BF, actor Max Ehrich, is now her fiance ... about 4 months after we all found out they were dating. Max popped the question on the sand in Malibu, and he came well-prepared ... with some major bling for her finger.

Seriously ... look at the rock.

Demi shared the big news late Wednesday night, saying ... "When I was a little girl, my birth dad always called me his 'little partner' -- something that might've sounded strange without his southern cowboy like accent. To me it made perfect sense. And today that word makes perfect sense again but today I'm officially going to be someone else's partner."

She says of Max -- "I knew I loved you the moment I met you," and adds ... "I’m ecstatic to start a family and life with you. I love you forever my baby. My partner. Here’s to our future!!!!"

Max dropped some poetry of his own, saying ... "You are every love song, every film, every lyric, every poem, everything I could ever dream of and then some in a partner in life. Words cannot express how infinitely in love with you I am forever and always and then some."

After the engagement ... the 2 hit up Nobu for a celebratory dinner.

Demi and Max started dating in March, right before stay-at-home orders went into effect for the coronavirus pandemic, and were spotted out a few times in masks grocery shopping.

They also had an isolated getaway last month in one of America's coolest vacation homes -- the "invisible house." It seems quarantining together has proven their relationship is the real deal.

Congrats!!