Marilyn Mosby, the State's Attorney for the city of Baltimore, joined us on "TMZ Live" Wednesday and told us how an experimental approach to limiting the spread of the virus is paying big dividends.

Under Mosby, Baltimore stopped prosecuting low-level offenses like drug possession and prostitution when the pandemic first hit ... and ever since, she says violent crime is down a whopping 20 percent.

Marilyn tells us there are other benefits, like fewer folks entering the criminal justice system and less crowded jails, and that's why Baltimore is making the changes permanent.

Now, the city's blazing a new trail ... Baltimore's partnering with a local behavioral health service to help folks in need seek treatment, instead of placing them in handcuffs.