The actress returned to Twitter last month after a lengthy absence following a a rehab stint for exhaustion in January.
Yesterday morning she told fans she'd have a new name "by the time I hit 5 mil followers," and she followed that up with her new moniker hours later.
She wrote, "So hard finding a name that was fun somewhat playful and available. So for now it will be @justdemi It could grow on me!"
Minutes later she added, "Thanks everyone for your help in finding my new name!"