Dominion Security was an agency of the Dominion which was responsible in some manner for both internal security and counter espionage. A parallel agency was Dominion Intelligence.
In 2374, after Gul Dukat had led Cardassia to join the Dominion, Dominion Security established a presence on Cardassia Prime. When Garek attempted to contact several former associates of the Obsidian Order in order to oppose the new government, within one day all of his contacts had been killed, apparently by Dominion Security. Garek remarked that Dominion Security was obviously very efficient and that one should "admire" such efficiency. (DS9: "In the Pale Moonlight")