Val Kilmer's health had seriously deteriorated near the end of his life ... leaving him unable to get out of bed for years before his death, TMZ has learned.

Family sources tell TMZ ... Val had a lack of energy related to his cancer treatment -- and, as a result, he was bed-bound for a long time.

While he was cancer-free, we're told his body simply couldn't keep fighting at the end ... and, his body eventually shut down.

According to our sources, Val was very frail near the end of his life ... and, his death wasn't sudden. His health seriously declined in the past week, and his friends and family came to Val's side at the hospital in the days before his death. He had been previously hospitalized earlier this year.

We are also told that a video of Kilmer donning a Batman mask he posted to Instagram in February was taken years ago... so, his appearance in it isn't accurate to how he looked in the last months of his life.

Kilmer hadn't appeared in public for years before his death ... last attending the Thespians Go Hollywood Gala back in 2019 with his daughter, Mercedes.

Val has two children ... and, we're told they're working on his funeral arrangements. As you know, Mercedes confirmed her dad's passing to the New York Times yesterday ... revealing he died after a battle with pneumonia.

Kilmer was hospitalized with throat cancer back in 2015 ... and, while he eventually went into remission, his voice was irreparably harmed by the disease.

Numerous celebs including Cher, Francis Ford Coppola and Jennifer Tilly have paid tribute to Kilmer in the hours after his death.

Val was 65.