Republican fundraiser Elliott Broidy on Monday sued a former high-ranking United Nations official, alleging he was an unregistered agent of Qatar who participated in a campaign against Mr. Broidy as part of an operation by the Persian Gulf state to silence its critics and influence U.S. policy.

Mr. Broidy alleges that at the direction of Qatari officials, former U.N. Under-Secretary-General Jamal Benomar, who served as special envoy to Yemen and a special adviser to former Secretary-General Ban Ki-moon, orchestrated the dissemination...