Rathgar is a suburb of Dublin in Ireland. It was originally a village which from 1862 was part of the township of Rathmines and Rathgar; it was absorbed by the growing city and became a suburb in 1930. It lies about three kilometres south of the city centre. Rathgar, in the Middle Ages, was a farm belonging to the Convent of St Mary de Hogges, at present-day College Green. 

At the Dissolution of the Monasteries, Rathgar was granted to the Segrave family: they built Rathgar Castle, ownership of which subsequently passed to John Cusacke, who was Lord Mayor of Dublin in 1608.  

The castle remained in the possession of the Cusack family for over a century, but gradually decayed and was a ruin by the end of the eighteenth century. No trace of it remains today, though it is thought to have been located at present day 44-49 Highfield Road. 

In 1649 the Duke of Ormonde commander of the Anglo-Irish Royalist army established his camp at Rathgar during the Siege of Dublin. He was then routed at the Battle of Rathmines nearby by English Republican forces under Michael Jones. The village began to develop in the eighteenth century. Rathgar Avenue may be the oldest street, while Highfield Road was developed in 1753. Zion Church and Christ Church Rathgar were built in the 1860s.