Weapons of Mass Destruction Prohibition Improvement Act of 2003

Weapons of Mass Destruction Prohibition Improvement Act of 2003 - Amends the Federal criminal code to expand the jurisdictional bases and scope of the prohibition against the use of, and threat, attempt, or conspiracy to use, weapons of mass destruction (WMD) to cover such actions against property within the United States under specified circumstances, including property owned, leased, or used by a foreign government.

Restores the coverage of chemical weapons under the prohibition.

Expands the categories of restricted persons subject to prohibitions relating to select agents to cover persons acting for or on behalf of, or operating subject to the direction or control of: (1) a government or official of a country that Secretary of State has determined has repeatedly provided support for acts of international terrorism; or (2) a terrorist organization.

Amends the Atomic Energy Act of 1954 to prohibit participation in the development or production of any special nuclear material or atomic weapon except as authorized by that Act.

Prohibits anyone within the United States or subject to U.S. jurisdiction from willfully participating in or providing material support or resources to a nuclear weapons program or other WMD program of a foreign terrorist power (or attempting or conspiring to do so). Establishes extraterritorial Federal jurisdiction over such offense.