1521 in science
- Francisco de Gordillo explores the Atlantic coast up to South Carolina.
- March 6 – Ferdinand Magellan discovers Guam in the Mariana Islands.
- March 16 – Magellan reaches the Philippines.
- April 7 – Magellan arrives at Cebu Island.
- November 8 – Magellan's expedition, now led by Juan Sebastián Elcano, arrives in the Maluku Islands.