Justin George Muzinich (born November 5, 1977, in New York City) is an American financier and former government official. Raised in New York City by his father, George Milan Muzinich, founder and president of Muzinich & Company (an international investment firm specializing in junk bonds), and his mother, Camille Muzinich, a docent at the Metropolitan Museum of Art, he attended Groton School in Groton, Massachusetts. He earned an AB (magna cum laude, Phi Beta Kappa) from Harvard College in 2000, an MBA (Baker Scholar) from Harvard Business School in 2005, and a JD (John M. Olin Fellow) from Yale Law School in 2007. Muzinich is married to Eloise Davis Austin (since October 2008); no children are publicly documented.

His early career included roles in the mergers and acquisitions group at Morgan Stanley and at EMS Capital. He served as president of Muzinich & Company prior to 2018 and created and taught a course on credit markets at Columbia Business School from 2014 to 2016. In politics, he was Policy Director for Jeb Bush's 2016 presidential campaign (2015-2016). During the Trump administration, he served as Counselor to the Secretary at the U.S. Department of the Treasury before becoming Deputy Secretary of the Treasury from December 12, 2018, to January 20, 2021.

Post-government, Muzinich returned to Muzinich & Company as CEO and member of the Board of Owners (2021-present). He was a Distinguished Fellow at the Council on Foreign Relations (2021-2023) and a Senior Fellow at Harvard Kennedy School (2022-2023). He serves on the Board of Trustees of NewYork-Presbyterian Hospital, the Board of Directors of the Council on Foreign Relations, and co-chairs a bipartisan CFR Task Force on economic security with Gina Raimondo and James Taiclet.

In June 2020, while Deputy Secretary, Muzinich transferred at least $60 million in Muzinich & Co. assets to his father (with an option to reclaim them post-government), approved by ethics offices. During the COVID-19 pandemic, the firm benefited from U.S. Treasury and Federal Reserve junk bond interventions, prompting conflict-of-interest concerns as reported by ProPublica.