"We regret to announce that Keith Emerson died last night at his home in Santa Monica, Los Angeles," read a statement on the band's Facebook page.
Emerson was considered one of the top keyboard players of the prog rock era.
He died of a self-inflicted gunshot wound to the head, Santa Monica police confirmed to the BBC.
His death was being investigated as a suicide, police added.
A police spokesman said Emerson's body was found in the early hours of Friday morning by his girlfriend Mari Kawaguchi at their flat in the Californian city.
Former bandmate Carl Palmer said: "I am deeply saddened to learn of the passing of my good friend and brother-in-music, Keith Emerson.
"Keith was a gentle soul whose love for music and passion for his performance as a keyboard player will remain unmatched for many years to come."
Inspired by Jimi Hendrix's theatrics with the electric guitar, Emerson was famous for his showmanship and outlandish on-stage performance style.
"That part of the act was something that just felt natural to do; something that allowed me be more expressive," he told Counterculture magazine.
ELP achieved an international following and were particularly popular in Britain and Japan. Several of the group's albums, including Tarkus, Trilogy, and Brain Salad Surgery entered the top five on the British chart.
Tarkus, released in 1971, featured an opening track lasting more than 20 minutes, inspired by the fictional Tarkus character - a half-tank, half-armadillo creature that would appear on stage at gigs.
Before ELP, Emerson was a member of The Nice, which formed in 1967 but disbanded three years later.
In later life he pursued a solo career and remained active in the music business. He was forced to call off a tour in 2010 due to abnormal growth in his colon, but had a tour of Japan scheduled for next month.
His last concert took place in July 2015 at the Barbican in London, where he performed alongside the BBC Concert Orchestra in a tribute to Robert Moog, the inventor of the Moog synthesizer.
He was born in Yorkshire in 1944.