Mid-bosses, also known as sub-bosses, middle bosses, and other names, are strong enemies with their own room or area and a high amount of life energy. Usually, mid-bosses are the only one of their kind in a stage, are found around the middle of the stage, are stronger than enemies, are weaker than bosses. However, there are some exceptions, such as mid-bosses that are fought two or more times (e.g. Nightmare Snake), optional (e.g. Pteranoid), omre health than the main boss (e.g. RT-55J), and fought after the main boss (e.g. Giganto Bole).

There is usually a difference between a mid-boss and a main boss, the mid-bosses does not have a health gauge in most games , or has a health gauge but no Warning message in the X4-X6 . However, some main bosses also do not have health gauge (e.g. Break Man), so the difference between the two is not absolute.

The big enemies are not to be confused with mid-bosses, although some enemies are somewhere between the two, such as Mole Borer.