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.

The family launched a GoFundMe as medical bills continue to mount ... and they’re asking fans to chip in whatever they can. Early support is already rolling in -- including a donation from actor and radio host Big Boy.