A long-range point from midfielder Michael Finneran in the 69th minute gave Roscommon a dramatic win in a tense match at Hyde Park.
Monaghan manager Malachy O'Rourke was strongly critical of referee Derek O'Mahoney after the game, but Roscommon manager John Evans praised his players for overhauling a three-point deficit with 10 minutes left.
The win keeps Roscommon’s slim promotion hopes alive but Monaghan, will still be promoted if they beat Antrim.
Monaghan failed to make their dominance of possession count in the first-half during which they hit seven wides.
Points from Paul Finlay and Darren Hughes opened up a four-point lead by the 48th minute but Monaghan failed to score again.
Roscommon trailed by three with eight minutes remaining but Donie Smith, Cathal Cregg pointed before Finneran hit the winner.