Andrew A. Weissmann (born March 17, 1958) is an American attorney and professor. He was an Assistant United States Attorney from 1991 to 2002, when he prosecuted high-profile organized crime cases. He served as chief of the Fraud Section in the Department of Justice (2015–2017) and as a lead prosecutor in Robert S. Mueller's Special Counsel's Office (2017–2019). He was in private practice at Jenner & Block in New York during two separate periods away from public service. He has taught at New York University School of Law, Fordham Law School, and Brooklyn Law School. He is currently a professor at the New York University School of Law.

In 2002, President George W. Bush appointed Weissmann as deputy director and then director of the Federal Bureau of Investigation's Enron Task Force. Weissman also served as General Counsel of the Federal Bureau of Investigation from 2011 to 2013, under President Barack Obama.

Starting in 2015, he became chief of the Criminal Fraud Section of the U.S. Department of Justice. In June 2017, he was appointed to a management role on the 2017 special counsel team headed by Robert Mueller. To assume that position, Weissmann took a leave from his Department of Justice post. The special counsel's investigation concluded in 2019. At that time, Weissmann returned to his practice in the private sector.

Weissmann, who is of Jewish heritage, was born in New York City, the son of Gerald Weissmann and Ann Weissmann (née Raphael) in 1958, and grew up there, attending the Ethical Culture Fieldston School. He subsequently attended Princeton University where he received a bachelor's degree in 1980. After a Fulbright scholarship to the University of Geneva, he received a Juris Doctor degree from Columbia Law School (1984). He then clerked for Judge Eugene Nickerson in the United States District Court for the Eastern District of New York.

In 1991, Weissmann was an assistant U.S. attorney in the U.S. Attorney's Office for the Eastern District of New York and he remained in that role until 2002. While at EDNY, Weissmann tried more than 25 cases, some of which involved members of the Genovese, Colombo, and Gambino crime families. He led the prosecution team in the Vincent Gigante case, in which Gigante was convicted.

From 2002 to 2005, Weissmann was deputy director, appointed by George W. Bush.

His work, during his assignment as director of the task force investigating the Enron scandal, resulted in the prosecution of more than 30 people for such crimes as perjury, fraud, and obstruction, including three of Enron's top executives: Andrew Fastow, Kenneth Lay, and Jeffrey Skilling. In a follow-up case in U.S. District Court, Weissmann also was successful, controversially, at arguing that auditing firm Arthur Andersen LLP had covered up for Enron. Weissmann was characterized as a "pitbull" by The New York Times during the Enron prosecution and some said he deployed "hard-nosed tactics and a 'win-at-all-costs' mentality". He had argued for the district judge to instruct the jury that they could convict the firm regardless of whether its employees knew they were violating the law, a court ruling that later was overturned by the Supreme Court. In Arthur Andersen LLP v. United States the court held that "the jury instructions failed to convey the requisite consciousness of wrongdoing". Nonetheless, the case had already resulted in the destruction of the firm.

After the special counsel completed its mandate, in 2005, Weissmann was special counsel again with Robert Mueller.