Samuel Monroe Jr. showed a glimmer of hope amid his serious health battle … but TMZ has learned the actor has experienced a severe downturn, as his condition continues to decline.

Samuel’s wife Shawna Stewart tells TMZ … the “Menace II Society” actor is back on life support, in a medically induced coma … and has required three blood transfusions in a matter of days, as his body is decimated by MRSA pneumonia and meningitis. He’s been placed back on dialysis.

The update is a devastating one for his family and friends … since just a few days ago, Sam seemed to be on the mend … taken off the ventilator and conscious as his blood pressure stabilized. But the hopeful moment did not last long.

TMZ broke the news … the actor has been hospitalized and on life support since late last month — and the prognosis has been bleak.

As we previously reported, Sam’s condition took a terrifying turn after he was diagnosed with MRSA pneumonia on top of meningitis he’d reportedly been battling for more than a year — following multiple misdiagnoses.