1941 - January 13 – All persons born in Puerto Rico since this day are declared U.S. citizens by birth, through U.S. federal law.

The 1941 law that conferred U.S. citizenship on individuals born in Puerto Rico was known as the Jones-Shafroth Act. It was signed into law by President Woodrow Wilson on March 2, 1917. Before this act, residents of Puerto Rico were considered statutory citizens, meaning they were subject to the jurisdiction of the United States but did not have full rights of citizenship.

The Jones-Shafroth Act granted U.S. citizenship to Puerto Ricans, which allowed them to travel freely between the island and the mainland, and to serve in the military. However, Puerto Rico remained a territory, and its residents did not have voting representation in Congress or the right to vote in presidential elections (although they could participate in primaries).

The act was named after its sponsors, Senator Wesley Jones of Washington and Representative William Atkinson Jones of Virginia, and it also established a bill of rights for Puerto Ricans and created a legislative assembly for the island.