We know the actor was admitted into a Los Angeles-area hospital Tuesday night, coming in through the emergency room. A witness tells us Kel appeared to be conscious and alert when he arrived.

The cause of Kel's illness is unclear, but we do know as of Wednesday, he was still in the hospital getting treatment. It's unknown when he'll be released or what his current condition is.

We reached out to Kel's reps, who did not comment on his situation.

Kel's social media pages are still posting despite his hospital stay -- but it's mostly promotional material -- including his work on "Hollywood Squares" and upcoming flick, "Good Burger 2" ... so it's likely the posts were preplanned.

Fans know Kel got his start in the Nickelodeon series, "All That" in the 90's ... before teaming up with "SNL" star Kenan Thompson for "Kenan & Kel" -- the 2 also starred in "Good Burger" in the middle of their comedy show success ... with a sequel to the burger film dropping later this month.