The atmosphere of Mars is the layer of gases surrounding Mars. It is primarily composed of carbon dioxide (95%), molecular nitrogen (2.85%), and argon (2%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen, and noble gases. The atmosphere of Mars is much thinner and colder than Earth's having a max density 20 g/m³ (about 2% of Earth's value) with a temperature generally below zero down to −60 °C. The average surface pressure is about 610 pascals (0.088 psi) which is 0.6% of the Earth's value.

The currently thin Martian atmosphere prohibits the existence of liquid water on the surface of Mars, but many studies suggest that the Martian atmosphere was much thicker in the past. The higher density during spring and fall is reduced by 25% during the winter when carbon dioxide partly freezes at the pole caps. The highest atmospheric density on Mars is equal to the density found 35 km (22 mi) above the Earth's surface and is ≈0.020 kg/m³. The atmosphere of Mars has been losing mass to space since the planet's core slowed down, and the leakage of gases still continues today.

The atmosphere of Mars is colder than Earth's owing to the larger distance from the Sun, receiving less solar energy and has a lower effective temperature, which is about 210 K (−63 °C; −82 °F). The average surface emission temperature of Mars is just 215 K (−58 °C; −73 °F), which is comparable to inland Antarctica. Although Mars's atmosphere consists primarily of carbon dioxide, the greenhouse effect in the Martian atmosphere is much weaker than Earth's: 5 °C (9.0 °F) on Mars, versus 33 °C (59 °F) on Earth due to the much lower density of carbon dioxide, leading to less greenhouse warming. Furthermore, the Martian atmosphere contains much less water vapor than Earth's atmosphere and water vapor is another important contributor to the greenhouse effect. The daily range of temperature in the lower atmosphere presents ample variation due to the low thermal inertia; it can range from −75 °C (−103 °F) to near 0 °C (32 °F) near the surface in some regions. The temperature of the upper part of the Martian atmosphere is also significantly lower than Earth's because of the absence of stratospheric ozone and the radiative cooling effect of carbon dioxide at higher altitudes.

Dust devils and dust storms are prevalent on Mars, which are sometimes observable by telescopes from Earth, and in 2018 even with the naked eye as a change in colour and brightness of the planet. Planet-encircling dust storms (global dust storms) occur on average every 5.5 Earth years (every 3 Martian years) on Mars and can threaten the operation of Mars rovers. However, the mechanism responsible for the development of large dust storms is still not well understood. It has been suggested to be loosely related to gravitational influence of both moons, somewhat similar to the creation of tides on Earth.

The Martian atmosphere is an oxidized atmosphere. The photochemical reactions in the atmosphere tend to oxidize the organic species and turn them into carbon dioxide or carbon monoxide. Although the most sensitive methane probe on the recently launched ExoMars Trace Gas Orbiter failed to find methane in the atmosphere over the whole of Mars, several previous missions and ground-based telescopes detected unexpected levels of methane in the Martian atmosphere, which may even be a biosignature for life on Mars. However, the interpretation of the measurements is still highly controversial and lacks a scientific consensus.

The mass and composition of the Martian atmosphere are thought to have changed over the course of the planet's lifetime. A thicker, warmer and wetter atmosphere is required to explain several apparent features in the earlier history of Mars, such as the existence of liquid water bodies. Observations of the Martian upper atmosphere, measurements of isotopic composition and analyses of Martian meteorites, provide evidence of the long-term changes of the atmosphere and constraints for the relative importance of different processes.

In general, the gases found on modern Mars are depleted in lighter stable isotopes, indicating the Martian atmosphere has changed by some mass-selected processes over its history. Scientists often rely on these measurements of isotope composition to reconstruct conditions of the Martian atmosphere in the past.

While Mars and Earth have similar ¹²C / ¹³C and ¹⁶O / ¹⁸O ratios, ¹⁴N is much more depleted in the Martian atmosphere. It is thought that the photochemical escape processes are responsible for the isotopic fractionation and has caused a significant loss of nitrogen on geological timescales. Estimates suggest that the initial partial pressure of N₂ may have been up to 30 hPa.