Nicholas F. Taubman calls corruption "the antithesis of democracy and rule of law," which is stifling growth and investment.
In a speech on Thursday prior to his departure, he calls for swift and equal justice for everyone as an effective way to fight endemic graft.
He also warns that Romania will not be competitive with its neighbors or Western Europe until it improves its roads and transportation networks.
However, Taubman praised Romania for staging a three-day NATO summit earlier this year and said an investment by U.S. carmaker Ford that created 9,000 jobs was successful.
Taubman is a political appointee of U.S. President George W. Bush.