The Romulan underground was an organization that existed within the Romulan Star Empire.

This group had existed for over two centuries and despised the atrocities of both pirates and the Praetor of the Romulan government. Their hope was to overthrow the military government in order to live a peaceful existence with the United Federation of Planets. This was because the groups members felt the same as their Vulcan cousins in that they were weary of constant warring which they believed were taking a toll upon their people. Thus, they sought to form a lasting peace and to accomplish this they had infiltrated various high ranking positions throughout the Romulan empire. Whilst these agents professed allegiance to the Praetor, they secretly served their group's interests and attempted to help to the best of their positions.

In 2269, Commander Tanirius was part of the underground but pretended to be a loyal intelligence agent of the Praetor who disguised herself as a Human female called Emma Saenz. During this year, she worked to save members of the crew of the USS Enterprise whereupon she revealed her identity to the Starfleet crew. (TOS novel: Mindshadow)

The name of the organization was not known as Commander Tanirius did not wish to reveal the name in fear that they might be discovered though it was referred to as an underground. Its possible that they are the parent or perhaps the same group that featured in Unification and sought reunification with their Vulcan brethren.
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