In abstract algebra, a module is indecomposable if it is non-zero and cannot be written as a direct sum of two non-zero submodules.

Indecomposable is a weaker notion than simple module (which is also sometimes called irreducible module): simple means "no proper submodule" ${\displaystyle N<M}$, while indecomposable "not expressible as ${\displaystyle N\oplus P=M}$".

A direct sum of indecomposables is called completely decomposable; this is weaker than being semisimple, which is a direct sum of simple modules.

A direct sum decomposition of a module into indecomposable modules is called an indecomposable decomposition.