A monorail (from "mono", meaning "one", and "rail") is a railway in which the track consists of a single rail or a beam.

Colloquially, the term "monorail" is often used to describe any form of elevated rail or people mover. More accurately, the term refers to the style of track.

The term monorail originated from German engineer Eugen Langen in 1897. Monorails are single-rail systems serving as a track for passenger or freight vehicles, often found in airport transfers and medium-capacity metros. Monorail vehicles can be similar to light rail vehicles and can be used alongside conventional metro systems. Monorails differ from trams and light rail systems by always being separated from other traffic and pedestrians, and they do not use pantographs. They offer some advantages over trains, buses, and automobiles, such as avoiding traffic jams and providing better views. Maglev systems like Transrapid and Linimo are considered monorails under the Monorail Society's beam-width criterion. Monorails have been featured in popular culture, including movies, television shows, and video games.

The first monorail prototype was created in Russia in 1820, and since then, various monorail designs have been proposed and implemented. Early monorails used a single metal rail or a gyroscopically balanced system. By the 1950s, monorails adopted larger beam- or girder-based tracks, with ALWEG straddle and SAFEGE systems emerging as popular designs. Despite their futuristic appeal, monorails struggled to compete with automobiles and other conventional transport systems.

From the 1980s onwards, Japan became a leader in monorail mass transit, with Tokyo Monorail being one of the world's busiest systems. China also began developing monorails in the late 2000s, including the world's largest and busiest system in Chongqing. Monorails have since found continued use in niche shuttle markets, amusement parks, and a growing number of cities exploring them as mass transit solutions. Modern mass transit monorails typically use developments of the ALWEG beam and tire approach, with some configurations adopted by maglev trains.

Modern monorails rely on a solid beam as the running surface and are divided into two classes: straddle-beam and suspended monorails. Straddle-beam monorails are more common, with trains straddling a steel or reinforced concrete beam. Suspended monorails, like the Wuppertal Schwebebahn in Germany, are less common. Most monorails are powered by electric motors, but diesel-powered systems exist as well. Magnetic levitation (maglev) trains like the German Transrapid are built as straddle-type monorails. Current monorails can efficiently switch between lines, with suspended monorails using flanges inside the beamway and straddle-beam monorails using moving apparatus on top of a platform. Rubber-tired monorails are designed to handle a 6% grade, similar to or greater than rubber-tired light rail or metro lines.