Megascale engineering (or macro-engineering) is a form of exploratory engineering concerned with the construction of structures on an enormous scale. Typically these structures are at least 1,000 kilometers in length—in other words, at least 1 megameter, hence the name. Such large-scale structures are termed megastructures.

In addition to large-scale structures, megascale engineering is also defined as including the transformation of entire planets into a human-habitable environment, a process known as terraforming or planetary engineering. This might also include transformation of the surface conditions, changes in the planetary orbit, and structures in orbit intended to modify the energy balance.

Astroengineering is the extension of megascale engineering to megastructures on a stellar scale or larger, such as Dyson spheres,Ringworlds, and Alderson disks.

Megascale engineering often plays a major part in the plot of science fiction movies and books. The micro-gravity environment of outer space provides several potential benefits for the engineering of these structures. These include minimizing the loads on the structure, the availability of large quantities of raw materials in the form of asteroids, and an ample supply of energy from the Sun. The capabilities to employ these advantages are not yet available, however, so they provide material for science fiction themes.