Jay-Z Rides NYC Subway -- BALL SO HARD

Multi-millionaire rap icon Jay-Z made his most shocking purchase ever this weekend in NYC -- a METROCARD -- riding the subway to Brooklyn with the common folk ... moments before his big concert at the Barclays Center.

Jay and his crew took the "R" train from Canal Street in Manhattan to Atlantic Avenue in Brooklyn on Saturday to mark his final show at the new Nets stadium before basketball season kicks off this month.