"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 hates that her kids lost their father. An official cause of death has not been determined.

Kiker's brother William and his mother Joyce also confirmed Doug's death to TMZ ... and William shared a heartfelt message to Facebook saying Doug will always be his baby bro.

The singer's death was 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."

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.

RIP