Guglielmi Castle, Italian national heritage castle on Isola Maggiore, Lake Trasimeno, Italy.

Guglielmi Castle, also called Isabella Castle, stands on the southern part of Isola Maggiore in Lake Trasimeno, Umbria. Built in 1887 as a summer residence for Senator Giacinto Guglielmi and his wife Isabella, it incorporates the 14th-century church of San Francesco, a Franciscan monastery and a Romanesque bell tower into a neo-Gothic design inspired by Miramare and Ludwig II's castles.

Senator Giacinto Guglielmi, Marquis of Vulci, built the castle in 1887 as a summer residence and named it after his wife Isabella. He bought the church of San Francesco and the Franciscan monastery with its Romanesque bell tower and had them incorporated into the new neo-Gothic building, modelled on Miramare and Ludwig II's castles; work lasted four years. The Guglielmis held a lavish inauguration in 1894 and kept the property until 1975; it passed to new owners in 1990. In February 1944 the prefect of Perugia made it a provincial internment camp for about thirty Jews, who were helped to escape by the islanders.

The castle is named after Isabella, wife of Senator Giacinto Guglielmi, to whom the residence was dedicated. In 1904 their daughter Elena Guglielmi introduced Irish point lacemaking among the fishermen's daughters; the craft still flourishes on Isola Maggiore today.

Isola Maggiore is reached by ferry from Tuoro sul Trasimeno or Passignano sul Trasimeno, with no other way to reach it. The castle itself is not open to the public, so it is worth checking the current situation before making the trip.

Although the castle is listed as national heritage, restoration work stopped in 2010 when a private company went bankrupt and left the project unfinished. Since then the building has been left to deteriorate, making it a rare case where protected status has not been enough to prevent ongoing decay.