The XM8 is a developmental U.S. military designation and project name for a lightweight assault rifle system that was under development by the United States Army from the late 1990s to early 2000s. The Army worked with the small arms manufacturer Heckler & Koch (H&K) to develop the system to its requirements in the aftermath of the OICW contract, for which H&K had been a subcontractor to ATK. Although there were high hopes that the XM8 would become the Army's new standard infantry rifle, the project was put on hold in April 2005, and was formally cancelled on October 31, 2005.

General Dynamics was involved in latter stages and H&K had plans to produce the rifle at a plant in Georgia. H&K was British-owned at the start of the project, but was later bought back by a group of German investors. Engineering work was done at facilities in the United States and Germany.