11:00 AM PT-- A source close to Kristoff tells us the actor had checked himself into UCLA Medical Center in January for depression issues ... and had just been released within the last few days.

We're told his depression was linked, in part, to his son, Julian's suicide years ago.

8:50 AM PT -- Kristoff's fiancée, model Kseniya Mikhaleva, reacted to his death, saying ... "How did it happen??? How??? Why did you leave so early???? and left me all alone ..."

Kristoff and Kseniya just got engaged 5 months ago.

TMZ also got the dispatch audio from the fire department, which responded to a call for an "unconscious individual" at Kristoff's home. You'll hear officials use the term "natural causes" -- that only means there was no foul play.

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Kristoff St. John, star of the long-running soap opera 'Young and the Restless,' was found dead Sunday ... TMZ has learned.

Law enforcement sources tell us one of Kristoff's friends went to check on him at the actor's home in San Fernando Valley, and found his body. Police and paramedics were alerted and responded to the house. We're told Kristoff was pronounced dead on the scene.

We're told there was no sign of foul play, however one source says alcohol might have played a role.

Kristoff has played Neil Winters on 'Y&R' since 1991 -- a role that earned him 9 Daytime Emmy Award nominations. He also won 10 NAACP Image Awards.

Kristoff has a tragic and troubled history with suicide. He and his ex-wife, boxer Mia St. John, lost their son, Julian, when he took his own life in 2014, while getting treatment at a mental health facility. Mia and Kristoff sued the facility for negligence. The suit was eventually settled.

In 2017, around the anniversary of Julian's suicide, Kristoff threatened to kill himself with a gun. Cops placed him under a 72-hour hold for psychiatric evaluation.

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Mia reacted to Kristoff's death with an emotional post on social media (which she has since deleted), and it's clear she is connecting his death to Julian's. She wrote, "THAT HOSPITAL KILLED OUR SON @TheArtofJulian THEN MY HUSBAND @kristoffstjohn1. THATS WHAT HAPPENED! THEY KILLED MY FAMILY"

After their son's death, Kristoff and Mia founded the El Saber es Poder (Knowledge is Power) Foundation to battle mental illness.

Kristoff was 52.

RIP