Downey Jr. says Jimmy was in a fatal car accident around 8 PM Wednesday night, calling it a "terrible and shocking tragedy." He says Jimmy was like a brother to him, an uncle to his kids and "beloved by all who experienced his singular character and wit."

RDJ's also thanking Jimmy for supporting him throughout his recovery from drug addiction, saying Jimmy stuck with him every step of the way.

Robert's not alone in mourning Jimmy ... his celebs pals, in and out of the MCU, are recalling what he meant to them.

Mark Ruffalo is thanking Jimmy "for always being real, always being there to lend a hand and showing the way of light forward to so many people."

Chris Pratt says Jimmy was "awesome" and Jack Black says he was a "good man" ... while Jeremy Renner called his death a "devastating loss" and Ryan Reynolds said it's "heartbreaking."

Rich was 52.