Armstrong steered the ball low past the advancing Richard O’Donnell following Ryan Kent’s fine pass to help the Reds record a season’s double over the rock-bottom Millers, who showed plenty of passion, but lacked quality in the final analysis.
It extended their winless streak against the Reds in Rotherham to almost thirty-five-and-a-half years, with their last victory over Barnsley in the league on home soil being way back in October 1982.
A well-contested derby saw both sides go close in the first period in an entertaining spectacle.
The Reds, who handed a debut to loan recruit Matty James - and also restored former Millers player Aidy White to their starting line-up - with Gethin Jones switching to right-back following James Bree’s exit to Aston Villa - almost conjured a breakthrough in eyecatching fashion on the half hour.
A lovely defence-splitting pass from new Reds captain Marc Roberts caught the Millers back four flat-footed and Adam Armstrong surged clear before seeing his dinked effort clip the top of the crossbar with Richard O’Donnell stranded outside of goal.
Six minutes later, the Millers knocked on wood when Joe Mattock’s angled low drive clipped the outside of the post after an excellent counter-attack.
The visitors generally had the better of proceedings and showed more poise on the ball, with White not far away with a cross-shot in the 20th minute.
But the Millers also had their moments with Angus MacDonald making a key block to deny Danny Ward, who lined up from the off despite speculation over his future, in the first quarter of an hour.
Ward fired the first salvo of the second period, testing the reactions of Davies, who tipped the striker’s instinctive curler over in the nick of time, with the Millers starting the second half with intent with Barnsley put under concerted pressure.
A rare counter saw Armstrong blaze wide, but after a strong spell from the hosts, the Reds started to crept back into proceedings and struck the decisive blow with twenty minutes left.
A searing break ended with Kent sending Armstrong clear with a fine throughball and he slotted the ball past O’Donnell in front of the massed away ranks.
The goal deflated the hosts and Barnsley went close to a second in the final ten minutes when Watkins’ ferocious strike was superbly tipped over by O’Donnell.
Rotherham United: O’Donnell; Fisher (Allan 77), Wood, Belaid (Kelly 25), Mattock; Forde (Bray 86), Smallwood, Vaulks, Taylor; Ward, Yates. Substitutes unused: Bilboe, Blackstock, Adeyemi, Warren.
Barnsley: Davies; Jones, MacDonald, Roberts, White (Evans 60), Watkins, Scowen, James, Kent (Hammill 87); Bradshaw, Armstrong (Williams 81). Substitutes unused: Townsend, Jackson, Moncur, Payne.
Referee: K Stroud (Hampshire).
Attendance: 11,050 (2,601 Barnsley supporters).