A disallowed goal in the second half from Armagh’s Ross McQuillan ultimately proved fatal, but Roscommon’s platform for victory was built in the first half.
A two-pointer from Diarmuid Murtagh in the opening minute set the tone as after Oisin O’Neill opened Armagh’s account at the other end, the Roscommon captain was put through for a third-minute goal.
It would get better for the hosts as Blaine Hughes’ subsequent kick-out went awry and Enda Smith was put in to thump home.
The sides traded a pair of scores before Murtagh kicked another two-pointer, while the crossbar denied Dylan Ruane a third goal for the hosts.
Still, they pushed further clear with a Daire Cregg free and a third Murtagh two-pointer – this time from a free – before Armagh briefly rallied with points from Cian McConville and Ross McQuillan.
However, a two-pointer from Cregg and a further two scores extended the gap to 14 before Armagh enjoyed a decent spell with three points, but a Cregg free on the stroke of half-time left Roscommon with a commanding 2-14 to 0-8 advantage.
Smith extended the gap straight after the break, but Armagh roared back with the next six points, including back-to-back two-pointers from Oisin O’Neill.
The Orchardmen thought they were right back in it in the 47th minute when McQuillan crashed home, but the goal was ruled out as his side had left less than four players in their own half and instead of a four-point gap, it was eight with Murtagh tapping over a free in front of the posts.
This four-point swing was ultimately crucial but so too was Roscommon’s third goal in the 61st minute.
With just six between them, substitutes combined as Jack Duggan sent a ball inside for Paul Carey to palm home, but Armagh were not done and two-pointers from O’Neill and McConville either side of a Tomas McCormack point brought them to within four.
They had a big goal chance with minutes to play when Jason Duffy fed inside to McConville, but he could only divert into the arms of Aaron Brady.
Armagh kept pressing, but a Greg McCabe shot whistled over at the death as they slipped to defeat with Roscommon making it back-to-back wins over Ulster opposition.