Saint Thomas of Guiana was a Roman Catholic diocese and suffragan of Caracas.
It was erected by Pius VI on 19 December 1791, and comprised the former state of Bermúdez, districts of Nueva Esparta and Guayana, and territories of Amazonas, Caura, Colón, Orinoco and Yuruary, in the south and east of Venezuela.
The Caribs were Christianized by the early Spanish Franciscan missionaries. The episcopal city, Ciudad Bolívar, was established in 1764 by two Jesuits under the governorship of Joaquín de Mendoza, on the right bank of the Orinoco, and called San Tomás de la Nueva Guayana; but owing to a narrowing of the river was commonly known as Angostura. It played an important part in the national history, and in February 1819 Simón Bolivar was elected president there by the Congress of Colombia; in his honor the city has been renamed Ciudad Bolivar.