John Stockton I COULD DUNK A BASKETBALL ... But Not Anymore ...

John Stockton is finally settling one of the great mysteries surrounding the NBA -- can he dunk a basketball????

The Hall-of-Famer was out in NYC yesterday promoting his new book," Assisted" when we asked if he ever got above the rim.

John's answer -- "Not anymore ... I could at one time."

For the record, trying to find a video clip of John dunking a basketball is like trying to find the Loch Ness monster ... it's virtually impossible.

So, here's the next best thing ...