January – February – March – April – May – June – July – August – September – October – November – December
- September 4, 1972 – Birth of Carlos Ponce, actor and singer
- September 6, 1964 – Birth of Rosie Perez, Academy Award-nominated actress and filmmaker
- September 7, 1934 – Birth of Juan "Pachín" Vicéns, basketball player (d. 2007)
- September 8, 1971 – Birth of Vico C (Armando Lozada Cruz), rapper and considered one of the founders of Latin hip hop
- September 9, 1898 – U.S. and Spanish Commissions met in San Juan to discuss the details of the withdrawal of Spanish troops and the cession of the island to the United States.
- September 9, 1939 – Birth of Bruce Gray, character actor
- September 12, 1983 – Los Macheteros assaulted the Wells Fargo depot located in West Hartford, Connecticut, stealing a total of seven million dollars, an event now known as Operation Aguila Blanca (White Eagle).
- September 13, 2004 – Death of Jose Juan Rodriguez Vargas Dorado, Puerto Rico
- September 15, 1931 – Birth of Raúl Dávila, actor, All My Children (d. 2006)
- September 15, 1933 – Birth of Henry Darrow, Emmy Award winning actor
- September 16, 1898 – Death of Ramón Emeterio Betances, main leader of the Grito de Lares and father of the Puerto Rican independence movement (b. 1827)
- September 17, 1922 – The Puerto Rican Nationalist Party was formed.
- September 17, 1937 – Birth of Orlando Cepeda, Mayor League Baseball Hall of Famer
- September 18, 1981 – Birth of Angelo Reyes, professional basketball player
- September 22, 1944 – The 65th Infantry landed in France and was committed to action on the Maritime Alps at Peira Cava.
- September 23, 1868 – The Grito de Lares occurred.
- September 23, 1911 – The College of Agriculture and Mechanic Arts was founded in Mayagüez.
- September 23, 2005 – Death of Filiberto Ojeda Ríos, after a stand-off with FBI agents (b. 1933)
- September 25, 1625 – The Dutch, under the leadership of Boudewijn Hendrick (Balduino Enrico), attacked San Juan, besieging Fort San Felipe del Morro and La Fortaleza.
- September 25, 1976 – Birth of Charlotte Ayanna, actress
- September 25, 1978 – Birth of Joel Piñeiro, Mayor League Baseball pitcher
- September 26, 1912 – The first school of advanced studies in Puerto Rico was established by Alonso Manso.
- September 27, 1932 – Hurricane San Ciprián struck the island, leaving 200-300 dead and more than 1,000 injured.
- September 30, 1946 – Birth of Héctor Lavoe, renowned salsa singer (d. 1993)
- September 30, 1972 – Roberto Clemente hits his 3,000th and final hit.