Barnsley lost at Fratton Park despite dominating for large spells against the team promoted as Champions with this result.
It was a lively start from Neill Collins' side and five minutes in they went ahead when good work from Conor Grant and then Adam Phillips saw Barnsley inside the box and Barry Cotter's cross-cum-shot was diverted home by the outstretched leg of Devante Cole.
But just two minutes later the hosts were level. Tino Anjorin's cross was deflected into the path of Kusini Yengi who rifled home from 10 yards.
John McAtee was sent through on goal on 20 minutes but his shot was repelled by Pompey 'keeper Will Norris. While Adam Phillips twice sent efforts narrowly wide in a first half dominated by the Reds.
HALF-TIME: Portsmouth 1-1 Barnsley
Just before the hour-mark, we went ahead again.
A run from Nicky Cadden saw Pompey defenders back off and he released McAtee whose first touch was sublime and with his second he put the Reds ahead.
But he would later bring down substitute Christian Saydee and fellow-sub Colby Bishop stuck home the spot kick for his fourth goal in four games against Barnsley.
And the visitors won it, to confirm promotion and the title when a corner kick was headed home by Conor Shaughnessy at the front post unopposed.
FULL-TIME: Portsmouth 3-2 Barnsley
Roberts, Cotter (O'Keeffe), Williams, de Gevigney, Earl, Cadden, Kane, Phillips, Grant (Jalo), McAtee (McCart), Cole.
Unused: Killip, Chapman, Cosgrove, Marsh.
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