In human anatomy, the extensor pollicis longus muscle (EPL) is a skeletal muscle located dorsally on the forearm. It is much larger than the extensor pollicis brevis, the origin of which it partly covers and acts to stretch the thumb together with this muscle.

The extensor pollicis longus arises from the dorsal surface of the ulna and from the interosseous membrane, next to the origins of abductor pollicis longus and extensor pollicis brevis.

Passing through the third tendon compartment, lying in a narrow, oblique groove on the back of the lower end of the radius, it crosses the wrist close to the dorsal midline before turning towards the thumb using Lister's tubercle on the distal end of the radius as a pulley.

It obliquely crosses the tendons of the extensores carpi radialis longus and brevis, and is separated from the extensor pollicis brevis by a triangular interval, the anatomical snuff box in which the radial artery is found.

At the proximal phalanx, the tendon is joined by expansions from abductor pollicis brevis and adductor pollicis.

The tendon is finally inserted on the base of the distal phalanx of the thumb.

6.7 to 9.7 centimetres (2.6 to 3.8 in) in length, the tendon passes through a long and superficial synovial sheath which, passing obliquely from the radial border of the forearm into the thumb, extends from the proximal border of the extensor retinaculum to the first carpometacarpal joint. In the synovial sheath a proximal and a distal mesotendon connect the tendon to the floor of the sheath.

Together with the tendons of the extensor pollicis brevis and the abductor pollicis longus, its tendon crosses the radial artery.