21st Century Buy American Act

21st Century Buy American Act - Authorizes the Secretary of Commerce to establish and carry out a program to award grants to any entity that is a manufacturer in the United States, is a firm certified as eligible to apply for adjustment assistance under the Trade Act of 1974, and is an entity that either: (1) mines, produces, or manufactures a nonavailable item; (2) is the last remaining manufacturer of an item in the United States and can prove hardship because of foreign competition; or (3) is the last remaining manufacturer of an item in the United States that is considered to be vital for national security purposes. Permits a recipient to use such a grant to: (1) increase its ability to compete for a government contract for, and to produce, a nonavailable item; (2) increase its capacity to produce items that are vital to national security and to create or retain jobs; (3) modernize or renovate manufacturing facilities using domestically made equipment; and (4) cover costs associated with obtaining access to adjustment assistance.

Defines a "nonavailable item" as an article that: (1) a federal agency has determined is not mined, produced, or manufactured in the United States in sufficient and reasonably available commercial quantities of a satisfactory quality; (2) has been subject to a waiver of buy American provisions under the American Recovery and Reinvestment Act of 2009; or (3) is on the list of nonavailable articles under the Federal Acquisition Regulation.

Amends the Buy America Act to: (1) make buy American requirements for articles for public use applicable without regard to whether the articles are for use outside the United States, if they are not needed on an urgent basis or are acquired on a regular basis; (2) require an analysis of the difference in the cost of such articles manufactured inside compared to outside the United States before a contract for such articles is entered; and (3) require an agency, before granting a public interest waiver or a waiver for use outside the United States, to consider the effect on domestic employment.

Requires: (1) each agency that applies an exception to the Buy American Act to notify the Director of the Office of Management and Budget (OMB), who shall post the notification on a publicly accessible website; and (2) the Comptroller General to report to Congress on the amount of articles purchased by each agency each fiscal year from entities that manufacture them outside the United States.