Apa posted a photo of Perry on a boat with the caption, "Rest in Love bro" Thursday night. The photo and message were the first we've heard from Apa since Perry's death on Monday.

Perry played Fred Andrews on the popular CW show ... Apa starred as Archie Andrews, Fred's son. Apa is among the countless other casts from the show to pay tribute to the late actor -- along with the writers and execs, who have dedicated the rest of the show's run to Perry

TMZ broke the story ... Perry died Monday at a hospital in Los Angeles surrounded by family, friends and loved ones. He suffered a massive stroke last Wednesday at his home and was rushed to the hospital where his condition never improved.

"Riverdale" paid tribute to Perry Wednesday night with a message at the end of the episode.

Perry was 52.

RIP