The "Joker" star's specifically calling for Gov. Andrew Cuomo to grant clemency to NY inmates, saying ... "The spread of coronavirus in prisons threatens the health and safety of all of us."

Phoenix adds that prisoners are unable to practice proper social distancing or maintain good hygiene, putting them at greater risk of infection, and says ... "No one deserves to die in prison from COVID-19."

It's unclear exactly what category of prisoner the actor's suggesting be released -- his message was delivered by the Release Aging People in Prison Campaign -- but Cuomo's office say they're already working on it.

Secretary to the Governor, Melissa DeRosa, tells TMZ ... a process has begun to release inmates "with 90 days or less remaining on their sentence who are 55 years of age or older, and whose underlying crime was not a violent felony or a sex offense."

This sounds similar to the "compassionate release" of qualifying prisoners Van Jones has been pushing for amid the pandemic ... meaning low-level and non-violent offenders.

Of course, many prisons across the country are already seeing high rates of coronavirus infection. Interestingly, one prison with ZERO cases is the supermax in Colorado that houses the most notorious criminals like El Chapo and the Unabomber.