The disco legend passed away earlier this week, according to founder of Chic Nile Rodgers ... who shared several throwback pics of the group together with "RIP Alfa Anderson. CHIC Organization" written on it.

He adds she is "forever loved" ... and did not include a cause of death in his post.

Alfa rose to prominence as one of the main vocalists for Chic ... joining the group in 1977 when legendary singer Luther Vandross encouraged her to audition.

She started out as a backup singer for the group ... but quickly moved up to a main singing role after Norma Jean Wright -- one of the OG singers -- decided to leave the group in 1978.

During her time as a co-main with Luci Martin, Chic dropped hit songs like "Le Freak," "Good Times" and "My Forbidden Lover" -- all of which Anderson played a major part in producing.

The original Chic broke up in 1983 ... and Anderson toured as a solo act with Vandross -- including playing legendary Wembley Stadium during this period.

Among the other big names she worked with ... Bryan Adams, Mick Jagger, Teddy Pendergrass, Jennifer Holliday and many more stars.

Alfa continued to record in her later years ... appearing on the song "I'll Be There" in 2015 after a new generation reformed Chic.

Anderson was also serious about education ... becoming a high school principal in the 1990s -- with her former school, El Puente Academy for Peace and Justice High School in Brooklyn writing a touching tribute to her.

Alfa was 78.