Sources at the prison tell us Kenney was spotted leaving the facility Wednesday morning with 76ers owner Michael Rubin, who's visited several times before... including Tuesday when he brought Patriots owner Robert Kraft to visit the incarcerated rapper.

According to the Mayor's spokeswoman ... Rubin invited Mayor Kenney, and the visit with Meek lasted about an hour. We're told Hizzoner believes the rapper could better serve the community outside prison.

We're told the Mayor believes Meek's case "makes clear and urgent the desperate need to continue working with our partners to reform our criminal justice system."

As we've reported ... Meek is serving 2 to 4 years for a probation violation -- the controversial sentence handed down by Judge Genece Brinkley.