Who can declare a law unconstitutional?

Any Article III (constitutional) court (or equivalent state court) can declare a law unconstitutional if the law is part of a case they're trying or reviewing; however, the government would probably appeal the case all the way to the Supreme Court to get a definitive answer.

The US Supreme Court is the ultimate arbiter of constitutionality, and has final authority over questions of constitutionality.

Article III Courts

• US District Courts
• US Court of International Trade
• US Court of Appeals Circuit Courts
• Supreme Court of the United States