A half marathon is a road running event of 21.0975 km (13 mi 192½ yd) — half the distance of a marathon. It is common for a half marathon event to be held concurrently with a marathon or a 5K race, using almost the same course with a late start, an early finish or shortcuts. If finisher medals are awarded, the medal or ribbon may differ from those for the full marathon. The half marathon is also known as a 21.1K or 13.1 miles, although these values are rounded and not formally correct.

A half marathon world record is officially recognised by the International Association of Athletics Federations. The official IAAF world record for men is 58:01, set by Geoffrey Kamworor of Kenya on September 2019 in Copenhagen, Denmark, and for women is 1:04:51, set by Joyciline Jepkosgei of Kenya on October 22, 2017, in Valencia, Spain. Participation in half marathons has grown steadily since 2003, partly because it is a challenging distance, but does not require the same level of training that a marathon does. In 2008, Running USA reported that the half marathon is the fastest-growing type of race.