Founded in 1847 in Klingenthal, Saxony, Germany, C.A. Seydel Söhne is the world's oldest harmonica manufacturer. The company was started by brothers Johann Christian and Christian August Seydel after mining declined in the Vogtland region, and it grew through the early twentieth century to employ hundreds of workers and produce millions of instruments per year. After decades operating behind the Iron Curtain as part of the East German state-owned harmonica industry, the firm was reorganized under new ownership in 2005 and has since continued to handcraft instruments at its original Klingenthal factory.

Seydel is best known for pioneering harmonicas with stainless steel reeds, which the company markets for their corrosion resistance and longer playing life compared to traditional brass. Its core lineup includes the 1847 series of diatonic harmonicas (Classic, Silver, Noble, and Lightning), the SESSION Steel and SESSION Standard diatonics, and a range of chromatic, tremolo, and orchestral models. The company also offers a custom shop, alternate tunings, and Low- and Solo-tuned variants aimed at players outside the standard Richter blues format.

The brand has a notable presence in blues and roots harmonica circles. Charlie Musselwhite is a longtime Seydel player and has a signature 1847 Lightning model, and the late James Cotton was also among the players associated with the company.

Seydel has 6 products cataloged on Equipboard, including Harmonicas. Their gear is featured by 1 artist, with the strongest followings in Blues. Notable users include James Cotton.