Pompey produced a dominant display to claim three points on a hot day in Cheltenham.
The 2-0 victory means that the Blues continue their unbeaten start to the 2022/23 campaign.
Colby Bishop opened the scoring after a Michael Jacobs delivery had been knocked down by Michael Morrison.
And Danny Cowley's side
secured all three points when Jacobs was brought down in the box and Bishop converted the penalty.