A gas ballon (also called a Charlier ballon for its inventor) is any balloon that stays aloft due to being filled with a gas less dense than air (such as helium or hydrogen. The first gas balloon made its flight in August 1789. It carried no passengers or cargo, and popped when it reached too high an altitude. Later that same year, a manned flight was made shortly after the first ascension in a hot-air balloon (and indeed the first recorded ascension by man in any flying device).

Gas balloons remained popular throughout the age before powered flight. They could fly higher and farther than hot-air balloons, but were more dangerous as they were usually filled with hydrogen gas (which, unlike helium, could be easily mass-manufactured). Gas balloons were used in the American Civil War, the Napoleonic Wars (to very limited extent), and throughout the 19th century by hobbists and show performers such as the Blanchards.

Today, gas balloons are filled with helium, which is much less volitale than hydrogen. Gas balloons are usually used for high-altitude research (such as with weather balloons) and for record-breaking manned balloon flights.

Helium-filled balloons for scientific research have flown to altitudes more than 50 km above sea level, above Earth's stratosphere and into the mesosphere. Such balloons fly above over 99.9% of Earth's atmosphere and operate in near-vacuum. They are used to image the Sun and stars in ultraviolet light that does not penetrate the atmosphere, to detect weak cosmic rays or the cosmic microwave background, or to study conditions at the top of the atmosphere.