A bassline (also spelled bass line) is the term used in many styles of popular music, such as jazz, blues, funk, and electronic music for the low-pitched instrumental part or line played by a rhythm section instrument such as the electric bass, double bass or keyboard (piano, Hammond organ, electric organ, or synthesizer). Basslines are in the low-pitched musical register, which is lower in pitch than the other musical parts (chords, instrumental melodies, and vocal melodies).

Basslines in popular music often use "riffs" or "grooves", which are usually simple, appealing musical motifs or phrases that are repeated, with variation, throughout the song. Bassline riffs usually emphasize the chord tones of each chord (usually the root note, the third note, the fifth note, and the octave of a chord), which helps to define a song's key. At the same time, basslines work along with the drum part and the other rhythm instruments to create a clear rhythmic pulse.

The type of rhythmic pulse used in basslines varies widely in different types of music. In swing jazz and jump blues, basslines are often created from a continuous sequence of quarter notes in a mostly scalar, stepwise part called a "walking bass line." In latin, salsa music, jazz fusion, electronica, and some types of rock and metal, basslines may be very rhythmically complex and syncopated. In bluegrass and traditional country music, basslines often emphasize the root and fifth of each chord. Minute rhythmic variations by the bass can dramatically change the feel of a song, even for a simple Groove" target="_blank">singer-songwriter groove.

Though basslines may be played by many different types of instruments and in a broad musical _range they are generally played on bass instruments and in the range roughly at least an octave and a half below middle C. In classical music, basslines play the same harmonic and rhythmic role; however, they are usually referred to as the "bass voice" or the "bass part."