A Ferris wheel (also known as an observation wheel or big wheel) is a nonbuilding structure consisting of a rotating upright wheel with passenger cars (sometimes referred to as gondolas or capsules) attached to the rim in such a way that as the wheel turns, the cars are kept upright, usually by gravity.

Some of the largest and most modern Ferris wheels have cars mounted on the outside of the rim, and electric motors to independently rotate each car to keep it upright. These wheels are sometimes referred to as observation wheels, and their cars referred to as capsules, however these alternative names are also sometimes used for wheels with conventional gravity-oriented cars.

The original Ferris Wheel was designed and constructed by George Washington Gale Ferris, Jr. as a landmark for the 1893 World's Columbian Exposition in Chicago. The term Ferris wheel later came to be used generically for all such structures, and Ferris wheels are now the most common type of carnival ride at state fairs in the US.

Since the original 1893 Chicago Ferris Wheel, there have been eight subsequent world's tallest-ever Ferris wheels. The current record holder is the 165-metre (541 ft) Singapore Flyer, which opened to the public in March 2008.