The double bass (), also known simply as the bass (), amongst other names, is the largest and lowest-pitched string instrument in the modern symphony orchestra (excluding unorthodox additions such as the octobass). Similar in structure to the cello, it has four, although occasionally five, strings.

The bass is a standard member of the orchestra's string section, along with violins, viola, and cello, as well as the concert band, and is featured in concertos, solo, and chamber music in Western classical music. The bass is used in a range of other genres, such as jazz, blues, rock and roll, rockabilly, country music, bluegrass, tango and folk music.

The bass is a transposing instrument and is typically notated one octave higher than tuned to avoid excessive ledger lines below the staff. The double bass is the only modern bowed string instrument that is tuned in fourths (like a bass guitar or viol), rather than fifths, with strings usually tuned to E₁, A₁, D₂ and G₂.

The instrument's exact lineage is still a matter of some debate, with scholars divided on whether the bass is derived from the viol or the violin family.

The double bass is played with a bow (arco), or by plucking the strings (pizzicato), or via a variety of extended techniques. In orchestral repertoire and tango music, both arco and pizzicato are employed. In jazz, blues, and rockabilly, pizzicato is the norm. Classical music and jazz use the natural sound produced acoustically by the instrument, as does traditional bluegrass. In funk, blues, reggae, and related genres, the double bass is often amplified.