Adams struck first for the high-flying hosts, turning in Theo Walcott’s cross in the first half to leave United trailing.
It was a largely dominant display from the Saints, who doubled their lead through Armstrong’s low finish late on to seal the points.
The result means Newcastle, who were beaten away from St. James’ Park for the first time since the start of the current Premier League season, missed the chance to move into the top half of the table ahead of this weekend’s remaining fixtures prior to the international break.
The Saints’ early opener was partly down to United’s sloppiness and partly down to some sharp finishing from Adams. The former Birmingham City frontman’s curling 20-yarder drew a superb stop from the in-form Karl Darlow, who tipped it away to his left.
But the hosts kept up the pressure and when Miguel Almirón hesitated before clearing, they pounced. Kyle Walker-Peters stole possession before Walcott picked up the ball on the right and fired a cross across the box which Adams took first time, thumping it beyond Darlow to give his side a seventh-minute lead.
Ralph Hasenhuttl’s men looked lively going forward but Newcastle, with the central defensive trio of Fabian Schär, Jamaal Lascelles and Federico Fernández who looked so steady in their last two outings, seemed to be coping with the intensity until they conceded an opportunity in the first half’s closing minutes. It was another swift break from Southampton, accelerated by Moussa Djenepo’s neat flick, and it led to Walcott cutting inside and shooting but with the bottom right-hand corner of Darlow’s net looking set to bulge, the former Arsenal man whipped his shot past the post.
The Saints started brightly after the break, too, with Walcott seeing a shot deflected past the upright to start a frantic few minutes. Then, from a James Ward-Prowse corner, Jannik Vestergaard nodded it down for defensive partner Jan Bednarek, but Lascelles was well-placed to clear it off the line. Finally, it was Oriol Romeu’s turn to have a go, but his fierce effort from the edge of the box was helped onto the woodwork by Darlow’s fingertips – a stunning save from the Magpies custodian.
Djenepo saw a penalty claim turned down after he tumbled following a challenge from Jacob Murphy, with Bruce introducing Matty Longstaff in place of Jeff Hendrick for his first appearance of the season, but the hosts continued to push. Darlow was tested again soon after, palming away Bednarek’s header from another Ward-Prowse corner, while a set piece at the other end proved one of United’s best chances of the contest, but Murphy’s free kick struck the wall.
In the end, the Magpies weren’t quite able to haul themselves back onto level terms. Joelinton and Andy Carroll were introduced from the bench in the final 15 minutes, just after Walcott had been denied a penalty following a last-ditch challenge from Lascelles, but Armstrong’s 81st-minute strike settled it. Sean Longstaff had a chance to clear but couldn’t get the ball under control, and Scotland international Armstrong picked it up and planted a low shot beyond Darlow to send Hasenhuttl’s side top of the table.
There was a final flurry of chances, with Joelinton’s header forcing a superb stop from Alex McCarthy and Darlow denying Armstrong at the death, but it remained 2-0.
