One Subject at a Time Act - Requires: (1) each bill or joint resolution to embrace no more than one subject, and (2) the subject to be clearly and descriptively expressed in the measure's title.

Prohibits an appropriations bill from containing any general legislation or change of existing law requirement, if its subject is not germane to the subject of such bill.

Declares void: (1) an entire Act or joint resolution if its title addresses two or more unrelated subjects, (2) provisions in legislation not clearly and descriptively expressed in the measure's title, (3) appropriation provisions in legislation outside the relevant subcommittee's jurisdiction, and (4) provisions of appropriation bills not germane to their subject matter.

Grants aggrieved persons and Members of Congress the right to bring an action against the United States to seek appropriate relief, including an injunction against the enforcement of any law the passage of which did not conform to this Act.