Maynooth Castle, a ruined 13th century castle in Maynooth, is a popular tourist attraction in County Kildare. The castle was built at the junction of two streams and became the home of the Fitzgerald family before its expansion by Sir John Fitzgerald in the 15th century.

The Fitzgeralds would later become the Earls of Kildare; Maynooth Castle was known as one of the largest and richest Earl's houses in Ireland. The original castle keep, constructed at the start of the 13th century, was one of the largest of its kind in Ireland.

Maynooth Castle provides an impressive entrance for Maynooth College, which was founded in 1795 and is famous for the education of the Irish Catholic priests. Today, the atmospheric castle lies in ruins—only the ruins of the keep and the main entrance gateway on the southern side remain.

Restoration work on the castle was undertaken by the Office of Public Works in 2000 in order to develop it into an Irish Heritage Site. An exhibition on the history of Maynooth Castle and the Fitzgerald family is on display at the on-site visitor centre.