A ha-ha (French: hâ-hâ or saut de loup), also known as a sunk fence, blind fence, ditch and fence, deer wall, or foss, is a recessed landscape design element that creates a vertical barrier (particularly on one side) while preserving an uninterrupted view of the landscape beyond from the other side.

The design can include a turfed incline that slopes downward to a sharply vertical face (typically a masonry retaining wall). Ha-has are used in landscape design to prevent access to a garden by, for example, grazing livestock, without obstructing views. In security design, the element is used to deter vehicular access to a site while minimizing visual obstruction.

The name "ha-ha" is thought to have stemmed from the reaction of the son of Louis XIV of France whose governess prevented him from approaching the drop for fear of injury. When he approached, he said "Ha Ha, this is what I'm supposed to be afraid of?" and since then more people started referring to "saut de loup" as "Ha Ha"; alternatively it could be just because of the element of surprise one has when approaching the construction. It may have been referred to as "ha-hah" as an abbreviation of "half and half" with half a wall and half a ditch although this explanation is rarely favored. In a letter to Daniel Dering in 1724, John Perceval, 1st Earl of Egmont (grandfather to the prime minister Spencer Perceval), observed of Stowe: "What adds to the beauty of this garden is, that it is not bounded by walls, but by a ha-hah, which leaves you the sight of the beautiful woody country, and makes you ignorant how far the high planted walks extend." In the 18th century, they were often called a sunken or sunk fence, at least in formal writing, as by Horace Walpole, George Mason and Humphry Repton. Walpole also referred to them as Kent-fences, named after William Kent.

George Washington called it both a "ha haw" and a deer wall.