A Micrometeoroid (also micrometeorite, micrometeor) is a tiny meteoroid; a small particle of rock from space, usually weighing less than a gram, that poses a threat to space exploration. The risk is especially high for objects in space for long periods of time, such as satellites. They also pose major engineering challenges in theoretical low-cost lift systems such as rotavators , space elevators, and orbital airships.

Micrometeoroids are extremely common in space, particularly near the Earth. Their velocities relative to a spacecraft in orbit can be on the order of kilometers per second, and resistance to micrometeoroid impact is a significant design challenge for spacecraft designers.

Micrometeoroids are typically small, typically metallic, pieces of rock broken off from larger chunks of rock and debris. They typically date back to the formation of the solar system. Since orbital velocities are so high, and since they can enter an earth orbit from any angle, micrometeoroids in earth orbit constantly intercept the orbits of spacecraft and impact them at high speed. While their tiny size limits the damage incurred, the high velocity constantly degrades the outer casing of spacecraft and, in the long term, can threaten the functionality of systems.

Micrometeoroids can also be easily found on earth in places where rainwater can concentrate them (such as a drain spout of roof gutters). Since metallic dust occurs relatively rarely on earth from other sources, metallic micrometeoroids can typically be separated from Earth dust via a strong magnet. Micrometeoroids comprise most of the 30,000 tons of space debris that are deposited on Earth every year.

Impacts by small objects with extremely high velocity are a current area of research in terminal ballistics. Accelerating objects up to such velocities is difficult; current techiques include linear motors and shaped charges.

In order to understand the micrometeoroid population better, a number of spacecraft (including Lunar Orbiter 1, Luna 3 and Mars 1) include micrometeoroid detectors.