The cellular structure unique to prokaryotic organisms is the nucleoid.

Here's why:

* Prokaryotes lack a true nucleus. Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.

* Eukaryotic cells, on the other hand, have a nucleus where their DNA is stored and protected within a membrane-bound compartment.

So, the nucleoid is a defining characteristic that distinguishes prokaryotes from eukaryotes.