In Star Trek, a protostellar nebula is a type of nebula that contains high levels of particle flux. This flux prevents starships from staying inside for more than 72 hours, but it also hides them from sensor scans. This phenomenon should not be confused with the other Star Trek phenomenon proto-nebulae, which appear to be newly-forming interstellar dust clouds. The term proto-stellar nebulae implies a nebula that is at a later stage of its life when it is forming proto-stars. The McAllister C-5 Nebula is an example of a protostellar nebula. (TNG: "Chain of Command, Part I").

The protostellar phase is an early stage in the process of star formation. For a solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a nebula, a protostar is formed by contraction out of the gas of giant molecular cloud in the nebula and the process ends with the formation of a T Tauri star, which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.