In architecture and construction, a sliding glass door (also patio door or doorwall) is a type of sliding door made predominantly from glass, that is situated in an external wall to provide egress and light. The doors can give access to a backyard or patio while providing a pleasant view, and when not fully covered can be a source of passive daylighting. Like a window, when open it also provides fresh air and copious natural light. It is considered a single unit consisting of two or more panel sections, some or all being mobile to slide open. Another design, a wall-sized glass pocket door has one or more panels movable and sliding into wall pockets, completely disappearing for a 'wide open' indoor-outdoor room experience.

The sliding glass door was introduced as a significant element of pre-war International style architecture in Europe and North America. Their predecessor is the sliding Shōji and Fusuma panel door in traditional Japanese architecture. The post-war building boom in modernist and Mid-century modern styles, and on to suburban ranch-style tract houses, multi-unit housing, and hotel-motel chains has made them a standard element in residential and hospitality building construction in many regions and countries.

"Handedness" of a sliding door is expressed as seen by an observer outside the building. When standing outside, a left-handed door opens from left to right (when closed, the handle is on the far left), and a right-handed door opens from right to left (when closed, the handle is on the far right). These relationships are sometimes described with the letters O and X, where O is the fixed panel and X is the sliding panel. The O/X notation allows the description of doors with more than two panels.

The traditional sliding doors design has two-panel sections, one fixed-stationary and one mobile to slide open. The actual sliding door is a movable rectangular framed sheet of window glass that is mounted parallel to a similar and often fixed similarly framed neighboring glass partition. The movable panel slides in a fixed track usually, and in its own plane parallel to the neighboring stationary panel.

A specialty form, for Washitsu or "Japanese-style rooms," creates sliding Shōji and Fusuma panel doors, with traditional materials for interior uses and contemporary adaptations for exterior exposure and uses. They are used in themed and contemporary restaurants, residences, Japanese garden tea houses, and other situations. Specialty manufacturers are located in Japan and Western countries

Another sliding doors design, glass pocket doors has all the glass panels sliding completely into open-wall pockets, totally disappearing for a wall-less 'wide open' indoor-outdoor room experience. This can include corner window walls, for even more blurring of the inside-outside open space distinction. Two-story versions are often electronically opened, using remote controls. For wide expanses, the opening point is centered, and three to six parallel tracks are used to carry the six to twelve sliding doors into the wall pockets on each side. Their recent popularity, shelter magazine coverage and technical and structural innovations have brought many options to market.

A third sliding doors design has all the glass panels suspended from above, leaving a trackless and uninterrupted floor plane. They also disappear into side pockets. On final closure, they slightly drop down to create a weatherproof seal. A German manufacturer developed the original technology, and its use is predominantly in temperate climates.

The sliding glass doors can be adapted to slide away from a corner connection leaving no corner post or framing in its wake. The corner stile is made up of two vertical profiles, a male and female section, which slot together and then slide away with the sliding doors. This meeting point does not have to be 90 degrees; it can also be an inverted corner allowing these frames to fit within any design seamlessly.