The bajo sexto (Spanish: "sixth bass") is a Mexican string instrument from the guitar family with 12 strings in six double courses.

It is played in a similar manner to the guitar, with the left hand changing the pitch with the frets on a fingerboard while the right hand plucks or strums the strings with or without a pick. Its original design was meant to accompany accordion players (and fiddlers) and thus offer a plethora of possibilities with only two musicians. This dynamic is a staple in regional Mexican styles, even with groups consisting of multiple instruments. The introduction of bass players eventually led to many players no longer using the lowest pair of strings. For that reason, the bajo sexto predates its close relative, the bajo quinto (Spanish: "fifth bass"), which eliminated the lowest pair of strings and only has 10 (five double courses).

When played in older styles of music where the instrument assumes the role of a bass, the strings are usually plucked with the fingers. In modern chordal and melodic styles, a pick is frequently used.

The history of the bajo sexto is characterized by its enigmatic origins. While limited historical documentation exists regarding its creation, compelling evidence suggests European influence. In particular, in the "mid-to-late 1800s", a German music store chain had branches throughout Mexico, including Oaxaca, part of a substantial migration of Germans to southern Texas during the 1830s and 1840s.

The musical styles and instruments of these German immigrants were embraced by Mexicans, who incorporated both into their own musical practices and traditions. Among the instruments that gained widespread popularity, the accordion emerged as one of the most influential. It found extensive use in dance music genres such as waltzes and polkas. The affordability of accordions made them accessible to a wide range of musicians, enabling them to explore Western music theory and develop their own interpretations of songs, ideal for solo performances.

While orchestras were popular in the early 19th century, they were far from common. The combination of accordions with bajo sextos introduced a versatile harmonic capability to folk musicians, allowing a simple duo to arrange and realize diverse styles of music, including popular songs which were previously the reserve of orchestras. The bajo sexto contributed a robust bass element, while establishing a firm rhythm that permitted the accordion to venture into intricate and lyrical melodic lines, often harmonized. This dynamic collaboration became a hallmark of Norteño music.

The addition of bass players provided bajo sexto players with opportunities to take turns leading melodic lines, enriching the texture of Mexican Norteño music. Many musicians relished the creative freedom afforded by bass players so much, they commissioned custom 10-string models, now recognized as the bajo quinto, which omitted the two lowest strings. This evolution underscores the adaptability and innovation within the realm of Mexican Norteño music. Norteño did not become a popular genre outside of the northern parts of Mexico until the 1950s. As groups began to record and perform for larger audiences, the style evolved as well.

The exact history of the bajo sexto is somewhat unclear. There are few written sources, and until very recently, most music dictionaries and encyclopedias did not mention the instrument. A few contemporary researchers have been working from oral sources—living players and luthiers—to trace the background of the instrument. They descend from the Spanish bandurrias and lutes that used double strings and were also tuned in fifths, perhaps to complete the harmonies in ensembles that required an instrument capable of giving the low notes of the harmonization of a melody.