The Mets selected Hartwig's contract from Triple-A Syracuse on Monday, Will Sammon of The Athletic reports.
Hartwig has collected a 4.21 ERA and 35:15 K:BB over 25.2 innings this season at Syracuse. The 25-year-old will be making his major-league debut when he gets into a game. Hartwig is taking the roster spot vacated by John Curtiss.