The Thayer Axial-Flow Valve is a replacement for the traditional rotary valve found on trombones with F attachments. Invented by Orla Ed Thayer in the 1980s, it was the biggest advance in the design of the trombone since the rotary valve was added (creating the F attachment) in the mid-1800s.

When engaged, rotary valves deflect air flow through the instrument at sharp angles, leading to stuffy response and audible tone color changes. The Thayer Valve, by contrast, does not deflect airflow by more than 25 degrees, and allows the tubing in the F attachment to be smoother.

The Thayer Valve kicked off an "arms race" among trombone designers and manufacturers that continues today (one perhaps fueled by the seemingly innate love of tinkering and gadetry much more characteristic of trombonists than players of other instruments, for whom tradition trumps design innovation). These newer valves have become the de-facto standard, especially among symphonic trombonists where consistent timbre is a highly-prized attribute.