Matthew P. Siben

Mr. Siben has 25 years of experience prosecuting high-profile securities fraud class actions on behalf of individual institutional investors.

After graduating from Harvard Law School in 2000, Mr. Siben joined the preeminent plaintiffs’ class action securities firm of the time, Milberg Weiss Bershad Hynes & Lerach. Starting his career in New York, Mr. Siben worked with founding father of the securities bar, Mel Weiss, to uncover the laddering scheme perpetrated by Wall Street banks to manipulate popular initial public offerings as the tech bubble burst.

In late 2001, Mr. Siben moved to San Diego to work with the foremost securities litigator of the period, Bill Lerach, and prosecute the most infamous securities fraud of the common era – Enron. Mr. Siben was influential in developing the scheme liability theories necessary to hold Enron’s investment bank partners liable for their role in falsifying Enron’s financial statements. Mr. Siben also had primary responsibility for prosecuting the case against Citigroup, obtaining a $2 billion settlement on behalf of the class.

In 2007, Mr. Siben joined litigation boutique Bernstein Litowitz Berger & Grossmann LLP to prosecute class cases and create the firm’s opt-out practice representing institutional investors in direct actions. Among other successes, Mr. Siben initiated and prosecuted the firm’s first opt-out action against Tyco International. Representing several mutual funds, a public pension fund, and hedge funds including Leon Cooperman’s Omega Partners, Mr. Siben settled the case against Tyco for $100 million. This settlement represented a significant multiplier over the recovery in the class case and continues to be a benchmark in opt-out litigation.

In 2009, Mr. Siben founded Dietrich Siben Thorpe LLP. Benchmark Litigation: The Definitive Guide to America’s Leading Litigation Firms & Attorneys wrote of the firm: “The litigators of Dietrich Siben Thorpe provide unparalleled opt-out representation of complex securities fraud claims. By selectively counselling individual investors in an effort to recover investment losses, the firm provides an alternative to maximize investor recoveries.” Mr. Siben successfully prosecuted numerous matters against “top-tier” defense firms, with most resolved on a confidential basis, such as Maverick Fund, LTD., et al. v. Mohawk Industries, Inc. et al., No. 21-cv-118 (N.D. Ga.) and Okumus Opportunistic Value Fund, LTD., et al. v. Valeant Pharma. Int., Inc., et al., No. 17-CV-6513 (D. N.J.). In rare instances, cases were not settled confidentially. For example, in a case alleging fraud against a solar panel manufacturer, Mr. Siben’s client Maverick Capital settled its action for $19 million, an excellent result relative to the class. See Smilovits v. First Solar, Inc., et al., No. 12-cv-555 (D. Az.) Dkt. No. 747 at 3. Similarly, in Antipodean Domestic Partners, LP v. Clovis Oncology, Inc., et al., Index No. 655908/2016 (Sup Ct, NY County), Mr. Siben successfully prosecuted an action alleging a biopharmaceutical company lied to investors about the efficacy of its primary pipeline oncology drug, reaching a $25 million settlement on behalf of a small hedge fund that opted out of the federal class action in Colorado to bring claims in New York state court.

Mr. Siben joined the Rosen Law Firm in January 2026, continuing his commitment to representing investors that have suffered as a result of corporate malfeasance.