Lovato, 27, and Ehrich, 29, are thought to have started dating just five months ago.
The singer confirmed the happy news by sharing a sweet photoshoot on Instagram, showing her and actor Ehrich kissing and embracing on the beach.
One shot showed off her impressive diamond engagement ring close up.
Lovato said that she was now “going to be someone else’s partner” and added that she knew she loved her fiance “from the moment I met you.”
She said that she felt “honoured” to accept his proposal and shared her excitement at starting “a family and a life” with Ehrich.
“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,” she wrote.
“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.
“@maxehrich -I knew I loved you the moment I met you. It was something I can’t describe to anyone who hasn’t experienced it firsthand but luckily you did too.”
The musician said she has “never felt so unconditionally loved by someone in [her] life” aside from her parents, “flaws and all” and explained that her fiance “never pressure[s her] to be anything other than [her]self.”
“And you make me want to be the best version of myself,” she continued.
“I’m honoured to accept your hand in marriage. I love you more than a caption could express but I’m ecstatic to start a family and life with you. I love you forever my baby. My partner. Here’s to our future!!!!”
Sharing new photos from the same shoot on Instagram, Ehrich described Lovato as “every love song, every film, every lyric, every poem, everything I could ever dream of and then some in a partner in life.”
He added: “Words cannot express how infinitely in love with you I am forever and always and then some. I cannot spend another second of my time here on Earth without the miracle of having you as my wife.”