Nightmare is a location in the Night Dimension where visitors go when they are having a nightmare. Wizeman the Wicked is the ruler of nightmares, having been the one who created it and its inhabitants, the Nightmarens. The player reaches Nightmare when they take on a Nightmaren boss. Nightmare is reachable in the Dream Gate by falling in the dark ocean or being kidnapped by Wizeman. If a Visitor falls into the dark ocean and Wizeman manages to steal their ideya, they will be trapped in Nightmare in their sleep forever.
- Sometimes, it is called the "Nightmare World".
- The world of Nightmare made an appearance as a vast warping DLC stage with Deadly Six Bonus Edition under the name "Nightmare Zone" in Sonic Lost World.