The USA FAA's definition of an ultralight is significantly different than that of most other countries and can lead to some confusion when discussing the topic.

The governing regulation in the United States is FAR 103, which specifies an "ultralight" as a single seat aircraft of less than 5 US gallons (19 L) fuel capacity, empty weight of less than 254 pounds (115 kg), a top speed of 55 knots (102 km/h), a maximum stall speed not exceeding 24 knots (45 km/h), and are only allowed to fly during daylight hours and over unpopulated areas. Weight allowances can be made for two-seat trainers, amphibious landing gear, and ballistic parachute systems.

In 2004 the FAA introduced the "Light-Sport Aircraft" category, which closely resembles other countries' Ultralight categories.

In the United States no license or training is required by law for ultralight aircraft, but training is highly advisable. For light-sport aircraft a sport pilot certificate is required, which is similar in requirements to other countries' Ultralight license.

In the U.S., a Sport Pilot certificate allows the pilot to operate a light-sport aircraft (a small, low-powered aircraft), under a limited set of flight conditions. The U.S. Sport Pilot certificate is similar to other countries' ultralight certificates. It is the only U.S. pilot certificate for powered aircraft that does not require a medical examination; a driver's license can be used as proof of medical competence.

On July 20, 2004 the FAA approved the Sport Pilot rule, to meet demand from recreational pilots flying small and experimental airplanes. The FAA worked in cooperation with the Experimental Aircraft Association to create the rule. This certification is easier to obtain than the private pilot certificate, and has more restrictions than the private certificate.

No FAA medical certificate is required. A valid and current driver's license certifies that the pilot is medically fit to fly. However, those who have had a FAA medical certificate revoked or denied may not use a driver's license, but rather must obtain a valid medical certificate.
A minimum of 20 hours of flight time including 15 hours of flight with an instructor and 5 hours of solo flight.
A passing score on an FAA knowledge test.
Pass a practical flight exam with an FAA-designated examiner.
Sport Pilots are only eligible to fly aircraft that are either certified as light-sport aircraft (LSA) or other certified aircraft that meet the criteria for light-sport aircraft.

Other restrictions placed on a holder of a Sport Pilot certificate are:

At most one passenger
Daytime flight only (civil twilight is used to define day/night)
No flight above 10,000 feet MSL
No flight in any of the airspace classes that require radio communication (classes A, B, C, or D) without first obtaining additional instruction and instructor endorsement
No additional ratings (such as an Instrument rating), although time in sport-light aircraft can be used towards the experience requirement of other ratings.
The U.S. definition of Microlight (Ultralight) Aircraft is much more restrictive than for 'Light Sport Aircraft' and no licence is required to fly a U.S.-defined Microlight.