Corey Feldman’s been nursing one serious grudge for years, and he’s finally letting it rip -- claiming that none other than Johnny Depp is the reason his Hollywood career took a nosedive.

On Billy Corgan’s "The Magnificent Others" podcast, Corey dropped a bombshell, saying he was originally cast as Arnie in 1993's "What’s Eating Gilbert Grape" -- and Depp pushed him out of the role, which then went to Leonardo DiCaprio.

Corey explained he was already cast in the role before Depp came into the picture. But Depp had more pull at the time and allegedly pulled producers aside, saying he didn’t want to work with him because he thought Feldman was a "junkie."

CF, who made his name as a child star in the '80s, had his struggles with drugs and alcohol and even went to rehab -- but by the time he was up for the role, he was sober and had turned his life around.

In fact, Corey says at the time he was trying to help Johnny's close pal, River Phoenix ... who was going through his own struggles, before collapsing and dying outside Depp's then co-owned nightclub, The Viper Room, from an overdose ... just weeks before 'Grape' opened.

Given all that, Corey’s still a bit bitter, saying he hasn’t even watched 'Grape' yet ... and has always wondered how his career would’ve panned out -- especially since DiCaprio snagged an Oscar nom for that role.