Established in the year 1600, Casiguran was the first missionary parish of Sorsogon. When the Spaniards first set foot in this part of Luzon in the 1570s, Casiguran was considered as their center of Kabikolan.
The name Casiguran is believed to come from the Bicol phrase kasi gurang, meaning, mostly old, pointing to the fact that it was the first parish and therefore to be considered to be populated by senior inhabitants.
It is also believed that the town was named Casiguran because, during the Spanish times, all the young men and ladies were kept hidden and only old people can be seen in the town. The reason of the townspeople for this is that the young people are being abused by the Spaniards, thus, kept them in hiding. One day, Spaniards set foot in the soil of the town and asked for its name. The people did not understand their language. Thinking that the Spaniards were asking why there are no young people, the people answered, "kasi gurang" (because only old people live there). The Spaniards thought that "Kasi gurang" was the name of the town. They left repeating the words "kasi gurang" as not to forget them, until it came to be called as "Casiguran."