The water-filled Castle Moat embracing three sides of the castle is one of its most memorable features.

The original moat, built in the 1440s by Sir Roger Fiennes, was considerably narrower. Much of the original moat eventually dried out, and sections were left dry for centuries. According to Christopher Hussey's December 1935 article in Country Life, a survey made in 1571 refers to the Moat as "...being of late drained for the more healthful standing of said house".

By the 20th century, the southern moat had become choked with wild vegetation that had begun to infiltrate the Castle walls, so extensive moat excavations were began in the late 1920's by Claude Lowther to resolve structural problems. During this process, the south end of the moat was widened considerably compared to the original design.

At the same time, the Castle's south moat bridge was also reconstructed and lengthened to span the wider moat. This inspired bit of landscape design has resulted in the iconic and much-photographed view of Herstmonceux Castle apparently floating on the water.

The historic moat along the north side of the castle was left dry. Remnants of the dry moat can be seen from the West Forecourt near Castle Reception, and from the pathway along the southern edge of the Elizabethan Garden.

Moats are depressions in the ground that surround city walls, castles, or other forts. Early moats were constructed when the earth was moved to form the walls that would surround the settlement. Early moats, in the form of ditches, have been found surrounding ancient towns and castles in ancient Assyria and Egypt. During the medieval period, these ditches were expanded and made more elaborate, evolving into what we now recognize as moats.

The main role of a moat is for security. Both dry and wet moats prevent easy access to the fortifications that they surround. Moats took on new significance as protection against siege weapons when masonry walls replaced earthen ones. During the 18th Century, moats provided obstacles against infantry attacks, although they were less effective as powerful firearms were developed.