A seven-string guitar is a guitar with seven strings instead of the usual six. Some types of seven-string guitars are specific to certain cultures (i.e. Russian and Brazilian guitars). The standard 7-string guitar tuning is BEADGbe. Seven-string electric guitars are used particularly in certain styles of music, such as heavy metal, rock and jazz. Rock and metal artists such as Born of Osiris, Devin Townsend, Chelsea Grin, Fear Factory, Steve Vai, Scribe, Dream Theater, Animals as Leaders, Bleeding Oath, Trivium, Korn, Limp Bizkit, Slayer, Megadeth, Deftones, Suicide Silence, Whitechapel, Behemoth, Periphery, Nevermore, TesseracT, Textures and Mucc have all experimented with seven-string guitars. Jazz artists such as George Van Eps, Bucky Pizzarelli, John Pizzarelli, Howard Alden, Lenny Breau and Jimmy Bruno use 7-strings.

Extra strings are usually added to extend the bass range of the 6-string guitar. These strings are commonly added in two different ways. The first and most common construction is to increase the width of the fingerboard such that the extra string (or strings) may be stopped by the left hand. The second method is to leave the fingerboard unchanged such that the extra bass strings lie next to the existing bass strings and free of the fingerboard in the same fashion in which the archlute and theorbo are constructed. Such unfrettable bass strings were historically known as diapasons or bourdons.