THD Electronics is a boutique guitar amplifier company founded in 1986 by Andrew "Andy" Marshall and based near Seattle, Washington. The company is widely credited with helping establish the American boutique tube amp market, beginning with its early reissue-style heads such as the 4-10 (A-10) and the Plexi. The name THD is an acronym for "total harmonic distortion," a nod to the contrast between hi-fi design goals and the saturated tones guitarists actually want — Marshall has also joked it stands for "Two Hairy Dogs," a reference to his bearded collies.

The brand is best known for the Hot Plate, a passive power attenuator introduced in 1994 that lets players push tube amps into saturation at lower volumes, and for Yellow Jacket tube converters, which allow amps designed for octal power tubes to run EL84s without rebiasing. THD's amplifier line includes the single-ended UniValve, the dual-output BiValve-30, and the Class AB Flexi-50, along with matching 2x12 extension cabinets. Products are hand-built in small runs and sold through the company directly as well as through dealers.

The company operates alongside Andy Marshall Designs, the workshop identity Marshall uses for ongoing design and build work. THD gear has been used by a range of professional guitarists across rock, blues, and metal, and the Hot Plate in particular has become a common sight in touring and studio rigs where cranked tube tone at manageable stage volumes is required.

THD has 16 products cataloged on Equipboard, including Guitar Power Attenuators, Guitar Amplifier Heads, and Guitar Amplifier Cabinets. Their gear is featured by 15 artists, with the strongest followings in Rock, Electronic, and Pop. Notable users include Noel Gallagher, Paul Gilbert, and Conor Curley.