An unindicted co-conspirator is one who is alleged in an indictment to have engaged in conspiracy, but is not charged in the same indictment. Prosecutors choose to name persons as unindicted co-conspirators for a variety of reasons including grants of immunity, pragmatic considerations, and evidentiary concerns.

The United States Attorneys' Manual generally recommends against naming unindicted co-conspirators, although their use is not generally prohibited by law or policy. Some commentators have raised due process concerns over the use of unindicted co-conspirators. Although there have been few cases on the subject, the Fifth Circuit Court of Appeals addressed these concerns in United States v. Briggs.

The term unindicted co-conspirator was familiarized in 1974 when then president Richard Nixon was named as an unindicted co-conspirator in indictments stemming from the Watergate Investigation.

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