21st Century Buy American Act

21st Century Buy American Act - Deems articles, materials, or supplies as made substantially all from articles, materials, or supplies mined, produced, or manufactured in the United States if the cost of the domestic components of such articles, materials, or supplies exceeds 60% of the total cost of all components of such articles, materials, or supplies.

Prohibits the use of the overseas exception to Buy American requirements unless one of the following criteria is met: (1) the articles, materials, or supplies are needed on an urgent basis; (2) the articles, materials, or supplies are to be  purchased using a local supplier; or (3) a cost analysis demonstrates that the articles, materials, or supplies to be acquired would be more than 50% more expensive if made in the United States.

Requires the head of a federal agency, in determining whether to apply the public interest exception to Buy American requirements, to: (1) consider the short-term and long-term effects of applying such exception on employment in the United States, and (2) determine that preserving or increasing employment in the United States is consistent with the public interest.

Requires each federal agency that applies an exception or grants a waiver to Buy American requirements to submit to the Director of the Office of Management and Budget (OMB) a notification of such exception or waiver. Requires the Director to post such notification on the OMB website.

Authorizes the Secretary of Defense (DOD) to establish and carry out a program to make or guarantee loans to certain business entities, up to $500,000 per entity, under the Defense Production Act. Specifies eligibility requirements for such entities, including requirements that such an entity is a manufacturer in the United States, is certified as eligible to apply for trade adjustment assistance, and meets criteria relating to the availability of an item.