The 85-year-old American singer — known for hits Killing Me Softly With His Song and The First Time Ever I Saw Your Face — is also struggling to speak due to the condition, also known as Lou Gehrig's disease.
Flack's manager said in a statement the disease, "has made it impossible to sing and not easy to speak.
"But it will take a lot more than ALS to silence this icon."
A documentary about Flack — who has won four Grammy awards and received 14 nominations — is set to premiere in New York later this month.
She is also due to publish a children's book in January.
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Her manager added she "plans to stay active in her musical and creative pursuits".
Amyotrophic lateral sclerosis (ALS) affects the nerve cells int he brain and spinal chord that control muscles, leading to slurred speech, twitching and failure to eat or breathe. It is also known as Lou Gehrig's disease after the baseball player who was diagnosed with the condition.
Flack suffered a stroke in 2016.
As well as Killing Me Softly — which was covered by The Fugees' Lauryn Hill in 1996 — Flack is known for songs Feel Like Makin' Love and The Closer I Get To You.
She rose to fame after her track The First Time Ever I Saw Your Face was used in the Clint Eastwood film Play Misty for Me.
Next year marks the 50th anniversary of her fourth album Killing Me Softly With His Song, released in 1973.
Flack said previously she wants her songs to be remembered as "classics" and and not just an "old hit".
The singer insisted: "I could sing any number of songs that I've recorded through the years, easily, I could sing them, but I'm going to pick those songs that move me.
"Now that's hard to do. To be moved, to be moved constantly by your own songs."