The scalenus medius, also known as the middle scalene muscle, is a neck muscle that spreads from the posterior tubercles (back sections) of the transverse processes (sides) of the six cervical (C2 through C7) vertebrae, which are located near the top of the spine, within the neck. It then inserts into the upper part of the first rib (via a broad attachment), at the back of the subclavian artery, under the collarbone.

The position of the middle scalene muscle, like the anterior scalene muscle, allows it to lift, or elevate, the first rib upward and to bend the neck sideways during the inhalation of gas into the lungs. Nerves that are part of the brachial plexus cervical nerves C3 to C8 — put this scalene muscle into action. The nerves passing through the middle scalene are the dorsal scapular and thoracic nerves.

The scalenus medius, the scalenus anticus, and the scalenus posticus muscles are all aid in inspiration (breathing in). However, instead of acting on the first rib, the scalenus posticus, or the posterior scalene muscle (the longest of the scalene muscles), lifts the second rib when air is being drawn into the lungs.