Lionel Richie remains hospitalized in a St. Louis intensive care unit as doctors check out his ticker ... TMZ has learned.

Sources familiar with the situation tell TMZ ... Lionel is undergoing testing for cardiac-related issues. We're told he's expected to be released from the hospital Wednesday.

As we first reported ... Lionel checked himself into the hospital for testing after his Saturday night performance at The Muny.

Sources initially told us doctors were also looking into possible mild dehydration, and Lionel was hoping to be discharged Tuesday.

Folks at that St. Louis show tell us Lionel appeared to be struggling toward the end of the concert, and he asked his team to bring him a chair so he could sit while performing his final song, "All Night Long."

Lionel was also rushed to a hospital in June after falling ill onstage in St. Paul, Minnesota ... which was the opening night of this tour. He told the crowd he felt "dizzy" and "strange" and cut the show short before paramedics took him to a hospital by ambulance.

That hospitalization forced him to postpone a Chicago tour stop. He's in the middle of a decent break as his next show isn't scheduled until September 26 in San Fran.