The Orange Is The New Black star wrote: "My whole heart and my whole soul is with @ddlovato today. I adore you and so does the entire world.
"You will fight this and you will come out of it even stronger. Addiction is powerful beyond measure, but you can defeat it. I love you."
Demi is currently recovering in hospital and is "stable and alert and breathing", according to E! News.
Her rep said in a statement to Variety: "Demi is awake and with her family who want to express thanks to everyone for the love, prayers and support.
"Some of the information being reported is incorrect and they respectfully ask for privacy and not speculation as her health and recovery is the most important thing right now."
Ruby's outpouring of support comes weeks after the two were caught flirting on Instagram.
Demi commented on Ruby's 4th of July photos with LOVE HEART EYE EMOJIS!
Then Ruby posted a leg selfie which she tagged Demi in.
Demi and Ruby aren't completely random. Back in 2013, claims came out that the pair actually dated for two WHOLE YEARS!
At the time, Ruby took to Instagram writing, "I'm like the one person Demi has slept with that didn't sell naked photos of her."
Demi responded to the rumours about to Complex, saying, "I did not have a relationship with Ruby Rose."