A bass amplifier (also abbreviated to bass amp) is a musical instrument electronic device that uses electrical power to make lower-pitched instruments such as the bass guitar or double bass loud enough to be heard by the performers and audience. Bass amps typically consist of a preamplifier, tone controls, a power amplifier and one or more loudspeakers ("drivers") in a cabinet.

While bass amps share many features with the guitar amplifiers used for electric guitar, they are distinct from other types of amplification systems, due to the particular challenges associated with low-frequency sound reproduction. This distinction affects the design of the loudspeakers, the size and design of the speaker cabinet and the design of the preamplifier and amplifier. Speaker cabinets for bass amps usually incorporate larger loudspeakers (e.g., 15 inches (380 mm) speakers are more common for bass than for electric guitar amps) or more speakers and larger cabinet sizes than those used for the amplification of other instruments. The loudspeakers themselves must also be sturdier to handle the higher power levels and they must be capable of reproducing very low pitches at high sound pressure levels.

The bassists who first sought methods to make their instruments louder were upright bass players. While the upright bass is a large instrument, standing about six feet tall (with its endpin extended), due to its low register it is not a loud instrument when played acoustically and because human hearing is less sensitive at low frequencies. In the 1890s and early 1900s, upright bass players performing in bars and brothels often found it difficult to be heard by the audience over louder instruments such as trumpet. A partial solution was playing slap bass style, slapping the strings against the fingerboard to make a relatively loud percussive sound.

In 1933, the Audiovox Manufacturing Company was founded by Paul Tutmarc, subsequently the inventor of the first electric bass, the fretted and solid-body Audiovox Model 736 Bass Fiddle, in 1936, which was designed to be played in a guitar-like horizontal manner. The instrument was sold with the first purpose-built bass amplifier, the Audiovox Model 936. Seen largely as a novelty, the few that were sold remained in the Seattle area.

The Ampeg Bassamp Company, founded in 1949 by Everett Hull, responded to the growing demand for electric bass equipment by producing a line of bass amplifiers. The first model offered was the Super 800, an 18-watt model with a single 12" speaker and a rear ventilation port. In 1951, Ampeg introduced a 20-watt version with a 15-inch speaker. In 1960, they introduced the B-15 Portaflex, a flip-top 25-watt tube bass amplifier with a single 15" speaker. While the Portaflex had a pleasing bass tone, and was used by studio bassists such as James Jamerson and Carol Kaye, it was not powerful enough to be used in a stadium or arena concert. Ampeg amplifiers were widely used by electric bass guitarists in the 1950s and 1960s.

Leo Fender resurrected the solid-body "bass guitar" in 1950 with the Fender Precision bass. Unlike the upright bass, a solid-body electric bass does not produce acoustic sound from a hollow body; while an upright bass player often benefits from using a bass amp, a bass amp is a necessity for an electric bass player.

By the late 1960s, as electric guitarists in rock bands began using powerful amplifiers to play large venues, bassists needed to keep up. The Acoustic 360 was a "200-watt, solid state head designed to drive the 361 cabinet, a rear-firing 18" speaker enclosure". The engineers who designed the amp and cabinet in 1967, Harvey Gerst and Russ Allee, mounted the 18" speaker in a folded horn enclosure; the 360 amp had a built-in fuzz bass effects unit. The Acoustic 360 and its 361 cabinet "...got the bass world ready for the Woodstocks, Altamonts and giant festival concerts" and it was used by notable players such as funk bassist Larry Graham, Led Zeppelin's bassist John Paul Jones and jazz fusion player Jaco Pastorius. John Paul Jones used two of the amp/cabs in Led Zeppelin; Dave Brown used them with Santana; John McVie played with the amp/cab in the beginning years of Fleetwood Mac. In December 1967, the loud sound of the Acoustic 360 led to The Doors getting "...arrested for noise violations".

Another 1960s-era amp and speaker that was used for loud, large venue performances was the Ampeg SVT (Super Vacuum Tube), a 300-watt amplifier head "powered by fourteen [vacuum] tubes" designed to be used with an 8x10" speaker cabinet.