The "Skyscraper" singer is one of the many stars who have shared both condolences and outrage over the tragedy, which left 50 dead and more than 50 injured after a gunman opened fire at the Pulse hotspot.
Lovato, 23, took to Twitter for the first time since the massacre, said she has been unable to find the words to express her sadness.
"There are no words for how upset I've been since Orlando," she wrote.
"I cry for the victims, ache for the families, and have immense admiration for the LGBTQ community and their resolve in absorbing this horror and uniting with strength and fearlessness. Stay strong. We stand with you."
She added more action needs to be taken to prevent a similar incident from happening in the future, writing, "We hurt. We mourn. We argue. We settle. We need to stop the cycle."
Lovato has long been a supporter of the LGBTQ community, and was recently feted for her advocacy with the coveted Vanguard Award at the GLAAD Media Awards.