"American Idol" alum Doug Kiker -- known by the moniker "The Singing Garbage Man" -- has died ... TMZ has learned.

Kiker died Monday, according to his ex-fiancée, Valerie Cook, who tells TMZ she does "hate" that her kids lost their father. An official cause of death has not been determined.

The singers death was also acknowledged by multiple posts from family members ... his sister Angela Evans, posted a GoFundMe -- saying the family needed help because "this was completely unexpected we are completely unprepared and are gonna need a little help getting him home from denver colorado and being able to give the memorial that not only he but anyone deserves."

Kiker's brother William also confirmed the death ... sharing a heartfelt message to Facebook saying he will always be his baby bro.

If you don't remember ... Kiker brought Katy Perry to tears on Rascal Flatts’ “Bless the Broken Road" back on Season 18 of the show, landing him in the Hollywood round.

He was eliminated in the following round after singing Marvin Gaye's "Ain't No Mountain High Enough."

Kiker was 32.