The law that was declared unconstitutional by the Supreme Court in Marbury v. Madison was a law called the Judiciary Act of 1789.
In this case, William Marbury had been appointed as a federal judge by John Adams after Adams had lost the 1800 presidential election to Thomas Jefferson. Marbury did not actually get his commission before Jefferson’s administration came into office. He sued to be put into the office to which he had been appointed. He claimed that the Judiciary Act of 1789 gave the Supreme Court original jurisdiction in the case.
The Court ruled that the Judiciary Act was unconstitutional because it broadened the Supreme Court’s original jurisdiction beyond what was set out in the Constitution.