Sergio Osmeña Sr. is a landlocked municipality in the coastal province of Zamboanga del Norte.
The municipality has a land area of 557.08 square kilometers or 215.09 square miles which constitutes 7.63% of Zamboanga del Norte’s total area. Its population as determined by the 2020 Census was 31,942. This represented 3.05% of the total population of Zamboanga del Norte province, or 0.82% of the overall population of the Zamboanga Peninsula region. Based on these figures, the population density is computed at 57 inhabitants per square kilometer or 149 inhabitants per square mile.