Thomastik-Infeld is a Vienna-based string manufacturer founded in 1919 by violin maker Dr. Franz Thomastik and civil engineer Otto Infeld. The company pioneered the steel instrument string as an alternative to traditional gut, and two years after its founding introduced the Spirocore, a flexible spiral rope core string that became a staple among upright bassists. In 1970, Thomastik-Infeld released the Dominant line of perlon-core strings, which broke into a market long dominated by gut and remains a standard reference for violin, viola, and cello players.

The company produces strings for bowed instruments (violin, viola, cello, double bass) as well as guitar and bass guitar. Notable product lines include Spirocore, Dominant, Versum, Rondo, Vision, Infeld, Peter Infeld (PI), Jazz Swing flatwound electric guitar strings, Jazz Flat Wound electric bass strings, and the Plectrum acoustic guitar series with bronze windings and silk filtering underlays. Manufacturing is done in-house in Vienna, with a proprietary precision rolling mill accurate to 0.5 micrometers.

Thomastik-Infeld strings are widely used across classical, jazz, and studio contexts, with the Jazz Swing flatwounds in particular holding a reputation for longevity and a warm, tempered top end favored by traditional jazz guitarists.

Thomastik-Infeld has 21 products cataloged on Equipboard, including Strings and Strings. Their gear is featured by 7 artists, with the strongest followings in Rock, Jazz, and Pop. Notable users include Mikael Akerfeldt, Joe Dart, and Pino Palladino.