POMPEY booked themselves into the group stages of the Premier League Cup with a magnificent display.
Noel Hunt rose at the far post to head home Conor Chaplin’s cross from the right in the 26th minute and from that point on there was going to be only one winner.
It was left to the outstanding Naismith, operating largely at left-back, to deservedly cap his display with a goal three minutes into time added on to seal a 2-0 triumph.
Championship side Barnsley were comfortably second-best throughout the Westleigh Park contest in front of a crowd of 355.
The scoreline should have been given far more breathing space by the hosts, who presented on-loan Dominic Hyam with his first appearance in a Blues shirt.
Nonetheless, they were full value for a victory which books them home and away fixtures against Norwich, Everton and Wolves in the group stages.
Chaplin and Naismith, substitutes for the first-team on Saturday, were both named in the line-up, while debutant Hyam partnered Matt Clarke in the centre of defence.
Noel Hunt, yet to start a Pompey match since his summer arrival, was also named in Foster’s side, as was the out-of-favour Bennett, an absentee from the last three Blues squads.
There were also outings for Ben Close, Liam O’Brien, Adam May and Ben Tollitt in a strong team to face the Championship Tykes.
When the game got underway, it took 11 minutes before a first booking was handed out, Naismith the culprit.
Featuring at left-back, his poor challenge on Rylan Mottley-Henry warranted the yellow card it attracted.
It was Naismith who also would provide the first attempt on goal on 18 minutes following a foul on Tollitt.
From 30-yards out, the Scotland curled in a left-foot free-kick which whistled past the wrong-side of the post with the keeper beaten.
Barnsley’s response was Omari Patrick latching onto a loose ball and dragging a shot from outside the box wide when perhaps there were better options.
In the Blues’ next attack, Naismith delivered a ball from his own half straight onto the head of Chaplin, whose header from the penalty spot was comfortably collected by the keeper.
The deadlock was broken on 26 minutes after Christian Oxlade-Chamberlain found Chaplin with a throw down the right.
The forward did the rest, getting to the byline to clip over a cross which was headed home at the far post by Hunt.
Moments later, Hunt fired in a shot from distance which Jack Walton did brilliantly to turn away with a flying stop.
From the resulting corner, Clarke found the ball at his feet and he swivelled to dig out an effort which flew straight into the arms of the Barnsley keeper.
The match was picking up and, on the half-hour mark, O’Brien was called into when he parried Mottley-Henry’s fierce drive.
With half-time approaching, the visitors wasted a glorious opportunity to level when Jarad Bird fired over from close range following an excellent delivery from Mottley-Henry down the right.
On 54 minutes Hunt should have increased the hosts’ lead when he fired into the sidenetting following an interchange of passing with Tollitt.
On 66 minutes, a wonderful run from Naismith saw him reach the left-sided byline and pull the ball back for Chaplin.
The shot was a clean one, but somehow Walton managed to touch it around the post when it appeared a goal was a certainty.
Pompey were by far the better side, yet that crucial second goal was continuing to elude as the half wore on.
Substitute Jez Bedford then flashed a shot across goal following excellent delivery down the left from Naismith.
On 83 minutes, Naismith floated in a cross from the left and Hunt planted his header wide of the post when he should have done far better with the opportunity.
Naismith then wrapped it up in stoppage time when he steered a left-foot shot past Walton from a tight angle to make it 2-0.
Pompey: O’Brien, Oxlade-Chamberlain (74 mins C.Davies), Hyam, Clarke, Naismith, Bennett, May, Close, Tollett (90 mins McDowell), Chaplin (78 mins Bedford), Noel Hunt. Subs Not Used: Hall, Bradbury.
Barnsley: Walton, Kebbie, Rodriguez, Cowgill, Templeton (59 mins Carvell), Mottley-Henry, Bird, Kay (74 mins Palmer), Charles (59 mins Bertie), Brown, Patrick. Subs Not Used: Proctor, Greatorex.